[hotfix][table] Fix incorrect error message of RootArgumentTypeStrategy

Author: dylanhz

flink-table/flink-table-common/src/main/java/org/apache/flink/table/types/inference/strategies/RootArgumentTypeStrategy.java

@@ -61,7 +61,7 @@ public Optional<DataType> inferArgumentType(
         if (Objects.equals(expectedNullability, Boolean.FALSE) && actualType.isNullable()) {
             return callContext.fail(
                     throwOnFailure,
-                    "Unsupported argument type. Expected nullable type of root '%s' but actual type was '%s'.",
+                    "Unsupported argument type. Expected NOT NULL type of root '%s' but actual type was '%s'.",
                     expectedRoot,
                     actualType);
         }

flink-table/flink-table-common/src/test/java/org/apache/flink/table/types/inference/InputTypeStrategiesTest.java

@@ -543,7 +543,7 @@ ANY, explicit(DataTypes.INT())
                         .calledWithArgumentTypes(DataTypes.VARCHAR(5))
                         .expectSignature("f(<VARCHAR NOT NULL>)")
                         .expectErrorMessage(
-                                "Unsupported argument type. Expected nullable type of root 'VARCHAR' but actual type was 'VARCHAR(5)'."),
+                                "Unsupported argument type. Expected NOT NULL type of root 'VARCHAR' but actual type was 'VARCHAR(5)'."),
                 TestSpec.forStrategy(
                                 "Logical type family instead of concrete data types",
                                 sequence(