[fix](nereids)modify split_part function to fold to null literal with correct data type (#42100)

pick from master https://github.com/apache/doris/pull/41109

## Proposed changes

Issue Number: close #xxx

<!--Describe your changes.-->

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

@@ -619,7 +619,7 @@ public class StringArithmetic {
                     return castStringLikeLiteral(first, "");
                 }
             }
-            return new NullLiteral();
+            return new NullLiteral(first.getDataType());
         } else if (number.getValue() < 0) {
             StringBuilder result = new StringBuilder(parts[Math.abs(number.getValue()) - 1]);
             return castStringLikeLiteral(first, result.reverse().toString());

regression-test/suites/nereids_p0/sql_functions/string_functions/test_split_part.groovy

@@ -30,4 +30,9 @@ suite("test_split_part") {
   }
 
   qt_1 "select split_part(k8, '1', 1), k8, split_part(concat(k8, '12'), '1', 1) from nereids_test_query_db.test order by k8 limit 2;"
+
+  explain {
+        sql("""verbose select split_part(replace_empty('we', "", "u"), ":", 5 - 5);""")
+        notContains "null_type"
+  }
 }