[hotfix][table] REGEXP_EXTRACT return type should always be nullable

This closes apache#17242.

flink-table/flink-table-common/src/main/java/org/apache/flink/table/functions/BuiltInFunctionDefinitions.java

@@ -629,7 +629,7 @@ ANY, and(logical(LogicalTypeRoot.BOOLEAN), LITERAL)
                                             logical(LogicalTypeFamily.CHARACTER_STRING),
                                             logical(LogicalTypeFamily.CHARACTER_STRING),
                                             logical(LogicalTypeRoot.INTEGER))))
-                    .outputTypeStrategy(nullableIfArgs(explicit(DataTypes.STRING())))
+                    .outputTypeStrategy(explicit(DataTypes.STRING().nullable()))
                     .build();
 
     public static final BuiltInFunctionDefinition FROM_BASE64 =

flink-table/flink-table-planner/src/main/java/org/apache/flink/table/planner/functions/sql/FlinkSqlOperatorTable.java

@@ -425,7 +425,7 @@ public void lookupOperatorOverloads(
                     SqlKind.OTHER_FUNCTION,
                     ReturnTypes.cascade(
                             ReturnTypes.explicit(SqlTypeName.VARCHAR),
-                            SqlTypeTransforms.TO_NULLABLE),
+                            SqlTypeTransforms.FORCE_NULLABLE),
                     null,
                     OperandTypes.or(OperandTypes.STRING_STRING_INTEGER, OperandTypes.STRING_STRING),
                     SqlFunctionCategory.STRING);

flink-table/flink-table-planner/src/test/java/org/apache/flink/table/planner/functions/StringFunctionsITCase.java

@@ -0,0 +1,51 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.apache.flink.table.planner.functions;
+
+import org.apache.flink.table.api.DataTypes;
+import org.apache.flink.table.functions.BuiltInFunctionDefinitions;
+
+import org.junit.runners.Parameterized;
+
+import java.util.Arrays;
+import java.util.List;
+
+import static org.apache.flink.table.api.Expressions.$;
+import static org.apache.flink.table.api.Expressions.call;
+
+/** Test String functions correct behaviour. */
+public class StringFunctionsITCase extends BuiltInFunctionTestBase {
+
+    @Parameterized.Parameters(name = "{index}: {0}")
+    public static List<TestSpec> testData() {
+        return Arrays.asList(
+                TestSpec.forFunction(BuiltInFunctionDefinitions.REGEXP_EXTRACT, "Check return type")
+                        .onFieldsWithData("22", "ABC")
+                        .testResult(
+                                call("regexpExtract", $("f0"), "[A-Z]+"),
+                                "REGEXP_EXTRACT(f0,'[A-Z]+')",
+                                null,
+                                DataTypes.STRING().nullable())
+                        .testResult(
+                                call("regexpExtract", $("f1"), "[A-Z]+"),
+                                "REGEXP_EXTRACT(f1, '[A-Z]+')",
+                                "ABC",
+                                DataTypes.STRING().nullable()));
+    }
+}