fix return type in extract multi

Signed-off-by: Jialiang Liang <jiallian@amazon.com>

Author: RyanL1997

core/src/main/java/org/opensearch/sql/expression/function/udf/RexExtractMultiFunction.java

@@ -31,9 +31,11 @@ public RexExtractMultiFunction() {
 
   @Override
   public SqlReturnTypeInference getReturnTypeInference() {
-    return call ->
-        call.getTypeFactory()
-            .createArrayType(call.getTypeFactory().createSqlType(SqlTypeName.VARCHAR, 2000), -1);
+    return call -> {
+      var elementType = call.getTypeFactory().createSqlType(SqlTypeName.VARCHAR, 2000);
+      return call.getTypeFactory()
+          .createArrayType(call.getTypeFactory().createTypeWithNullability(elementType, true), -1);
+    };
   }
 
   @Override

core/src/test/java/org/opensearch/sql/expression/function/udf/RexExtractMultiFunctionTest.java

@@ -160,6 +160,19 @@ public void testReturnTypeInference() {
     assertNotNull(function.getReturnTypeInference(), "Return type inference should not be null");
   }
 
+  @Test
+  public void testNullReturnTypeCompatibility() {
+    List<String> result1 =
+        RexExtractMultiFunction.extractMultipleGroups("no match", "(\\d+)", 1, 5);
+    assertNull(result1, "Should return null when no matches found");
+
+    List<String> result2 = RexExtractMultiFunction.extractMultipleGroups("abc", "(\\w+)", 2, 5);
+    assertNull(result2, "Should return null for invalid group index");
+
+    List<String> result3 = RexExtractMultiFunction.extractMultipleGroups("", "(\\w+)", 1, 5);
+    assertNull(result3, "Should return null for empty string with no matches");
+  }
+
   @Test
   public void testOperandMetadata() {
     assertNotNull(function.getOperandMetadata(), "Operand metadata should not be null");