SQL: Teach the CLI to ignore empty commands

Cause the CLI to ignore commands that are empty or consist only of
newlines. This is a fairly standard thing for SQL CLIs to do.

It looks like:
```
sql> ;
sql>
   |
   | ;
sql> exit;
Bye!
```

I think I *could* have implemented this with a `CliCommand` that throws
out empty string but it felt simpler to bake it in to the `CliRepl`.

Closes elastic#30000

x-pack/plugin/sql/sql-cli/src/main/java/org/elasticsearch/xpack/sql/cli/CliRepl.java

@@ -56,6 +56,11 @@ public void execute() {
                 multiLine.setLength(0);
             }
 
+            // Skip empty commands
+            if (line.isEmpty()) {
+                continue;
+            }
+
             // special case to handle exit
             if (isExit(line)) {
                 cliTerminal.line().em("Bye!").ln();

x-pack/plugin/sql/sql-cli/src/test/java/org/elasticsearch/xpack/sql/cli/CliReplTests.java

@@ -38,6 +38,28 @@ public void testBasicCliFunctionality() throws Exception {
         verifyNoMoreInteractions(mockCommand, mockSession);
     }
 
+    /**
+     * Test that empty commands are skipped. This includes commands that are
+     * just new lines.
+     */
+    public void testEmptyNotSent() {
+        CliTerminal cliTerminal = new TestTerminal(
+                ";",
+                "",
+                "",
+                ";",
+                "exit;"
+        );
+
+        CliSession mockSession = mock(CliSession.class);
+        CliCommand mockCommand = mock(CliCommand.class);
+
+        CliRepl cli = new CliRepl(cliTerminal, mockSession, mockCommand);
+        cli.execute();
+
+        verify(mockCommand, times(1)).handle(cliTerminal, mockSession, "logo");
+        verifyNoMoreInteractions(mockSession, mockCommand);
+    }
 
     public void testFatalCliExceptionHandling() throws Exception {
         CliTerminal cliTerminal = new TestTerminal(