[Fix](Nereids) fix append_trailing_char_if_absent function return null (

#40820)

example: select append_trailing_char_if_absent('it','a') would return
null in original design, it can not return null when folding constant on
fe any time

fe/fe-core/src/main/java/org/apache/doris/nereids/trees/expressions/functions/executable/ExecutableFunctions.java

@@ -28,9 +28,7 @@
 import org.apache.doris.nereids.trees.expressions.literal.LargeIntLiteral;
 import org.apache.doris.nereids.trees.expressions.literal.NullLiteral;
 import org.apache.doris.nereids.trees.expressions.literal.SmallIntLiteral;
-import org.apache.doris.nereids.trees.expressions.literal.StringLikeLiteral;
 import org.apache.doris.nereids.trees.expressions.literal.TinyIntLiteral;
-import org.apache.doris.nereids.trees.expressions.literal.VarcharLiteral;
 import org.apache.doris.nereids.types.DoubleType;
 
 import java.math.BigInteger;
@@ -105,15 +103,6 @@ public static Expression acos(DoubleLiteral literal) {
         }
     }
 
-    @ExecFunction(name = "append_trailing_char_if_absent")
-    public static Expression appendTrailingIfCharAbsent(StringLikeLiteral literal, StringLikeLiteral chr) {
-        if (literal.getValue().length() != 1) {
-            return null;
-        }
-        return literal.getValue().endsWith(chr.getValue()) ? literal
-                : new VarcharLiteral(literal.getValue() + chr.getValue());
-    }
-
     @ExecFunction(name = "e")
     public static Expression e() { // CHECKSTYLE IGNORE THIS LINE
         return new DoubleLiteral(Math.E);

fe/fe-core/src/main/java/org/apache/doris/nereids/trees/expressions/functions/executable/StringArithmetic.java

@@ -794,6 +794,9 @@ public static Expression urlDecode(StringLikeLiteral first) {
      */
     @ExecFunction(name = "append_trailing_char_if_absent")
     public static Expression appendTrailingCharIfAbsent(StringLikeLiteral first, StringLikeLiteral second) {
+        if (second.getValue().length() != 1) {
+            return new NullLiteral(first.getDataType());
+        }
         if (first.getValue().endsWith(second.getValue())) {
             return first;
         } else {

regression-test/suites/nereids_p0/expression/fold_constant/fold_constant_string_arithmatic.groovy

@@ -197,6 +197,7 @@ suite("fold_constant_string_arithmatic") {
 
     testFoldConst("select append_trailing_char_if_absent('a','c')")
     testFoldConst("select append_trailing_char_if_absent('ac','c')")
+    testFoldConst("select append_trailing_char_if_absent('it','a')")
 
     testFoldConst("select ascii('1')")
     testFoldConst("select ascii('a')")
@@ -684,4 +685,91 @@ suite("fold_constant_string_arithmatic") {
     // fix problem of cast date and time function exception
     testFoldConst("select ifnull(date_format(CONCAT_WS('', '9999-07', '-00'), '%Y-%m'),3)")
 
+    // Normal Usage Test Cases
+
+    // Test Case 1: Append missing trailing character
+    testFoldConst("select append_trailing_char_if_absent('hello', '!')")
+    // Expected Output: 'hello!'
+
+    // Test Case 2: Trailing character already present
+    testFoldConst("select append_trailing_char_if_absent('hello!', '!')")
+    // Expected Output: 'hello!'
+
+    // Test Case 3: Append trailing space
+    testFoldConst("select append_trailing_char_if_absent('hello', ' ')")
+    // Expected Output: 'hello '
+
+    // Test Case 4: Empty string input
+    testFoldConst("select append_trailing_char_if_absent('', '!')")
+    // Expected Output: '!'
+
+    // Test Case 5: Append different character
+    testFoldConst("select append_trailing_char_if_absent('hello', '?')")
+    // Expected Output: 'hello?'
+
+    // Test Case 6: String ends with a different character
+    testFoldConst("select append_trailing_char_if_absent('hello?', '!')")
+    // Expected Output: 'hello?!'
+
+    // Edge and Unusual Usage Test Cases
+
+    // Test Case 7: Input is NULL
+    testFoldConst("select append_trailing_char_if_absent(NULL, '!')")
+    // Expected Output: NULL
+
+    // Test Case 8: Trailing character is NULL
+    testFoldConst("select append_trailing_char_if_absent('hello', NULL)")
+    // Expected Output: NULL
+
+    // Test Case 9: Empty trailing character
+    testFoldConst("select append_trailing_char_if_absent('hello', '')")
+    // Expected Output: Error or no change depending on implementation
+
+    // Test Case 10: Trailing character is more than 1 character long
+    testFoldConst("select append_trailing_char_if_absent('hello', 'ab')")
+    // Expected Output: Error
+
+    // Test Case 11: Input string is a number
+    testFoldConst("select append_trailing_char_if_absent(12345, '!')")
+    // Expected Output: Error or '12345!'
+
+    // Test Case 12: Trailing character is a number
+    testFoldConst("select append_trailing_char_if_absent('hello', '1')")
+    // Expected Output: 'hello1'
+
+    // Test Case 13: Input is a single character
+    testFoldConst("select append_trailing_char_if_absent('h', '!')")
+    // Expected Output: 'h!'
+
+    // Test Case 14: Unicode character as input and trailing character
+    testFoldConst("select append_trailing_char_if_absent('こんにちは', '!')")
+    // Expected Output: 'こんにちは!'
+
+    // Test Case 15: Multibyte character as trailing character
+    testFoldConst("select append_trailing_char_if_absent('hello', '😊')")
+    // Expected Output: 'hello😊'
+
+    // Test Case 16: Long string input
+    testFoldConst("select append_trailing_char_if_absent('This is a very long string', '.')")
+    // Expected Output: 'This is a very long string.'
+
+    // Error Handling Test Cases
+
+    // Test Case 17: Invalid trailing character data type (numeric)
+    testFoldConst("select append_trailing_char_if_absent('hello', 1)")
+    // Expected Output: Error
+
+    // Test Case 18: Invalid input data type (integer)
+    testFoldConst("select append_trailing_char_if_absent(12345, '!')")
+    // Expected Output: Error or '12345!'
+
+    // Test Case 19: Non-ASCII characters
+    testFoldConst("select append_trailing_char_if_absent('Привет', '!')")
+    // Expected Output: 'Привет!'
+
+    // Test Case 20: Trailing character with whitespace
+    testFoldConst("select append_trailing_char_if_absent('hello', ' ')")
+    // Expected Output: 'hello '
+
+
 }