fix: allow QueryMessage with empty query string (#113)

PostgreSQL accepts QueryMessages with an empty query string. PGAdapter
now also accepts these and sends back a NoDataResponse without sending
the statement to Spanner.

src/main/java/com/google/cloud/spanner/pgadapter/statements/IntermediateStatement.java

@@ -411,11 +411,14 @@ private void executeSingleStatement(int index) {
         executionStatus = ExecutionStatus.AUTOCOMMIT;
       }
     }
-    try {
-      StatementResult result = this.connection.execute(Statement.of(statement));
-      updateResultCount(index, result);
-    } catch (SpannerException e) {
-      handleExecutionExceptionAndTransactionStatus(index, e);
+    // Ignore empty statements.
+    if (!"".equals(statement)) {
+      try {
+        StatementResult result = this.connection.execute(Statement.of(statement));
+        updateResultCount(index, result);
+      } catch (SpannerException e) {
+        handleExecutionExceptionAndTransactionStatus(index, e);
+      }
     }
   }
 

src/test/java/com/google/cloud/spanner/pgadapter/JdbcSimpleModeMockServerTest.java

@@ -92,6 +92,18 @@ public void testQuery() throws SQLException {
     assertTrue(request.getTransaction().getSingleUse().hasReadOnly());
   }
 
+  @Test
+  public void testEmptyStatement() throws SQLException {
+    String sql = "";
+
+    try (Connection connection = DriverManager.getConnection(createUrl())) {
+      assertFalse(connection.createStatement().execute(sql));
+    }
+
+    // An empty statement is not sent to Spanner.
+    assertEquals(0, mockSpanner.countRequestsOfType(ExecuteSqlRequest.class));
+  }
+
   @Test
   public void testWrongDialect() {
     // Let the mock server respond with the Google SQL dialect instead of PostgreSQL. The