[INLONG-12213][SDK][Transform] Support parsing SQL NULL literal via NullValueParser

[luchunliang]

Fixes #12213

Motivation

Modifications

Proposed Change

Introduce a NullValueParser following the existing StringParser pattern:

@TransformParser(values = NullValue.class)
public class NullValueParser implements ValueParser {

    public NullValueParser(NullValue expr) {
        // The NULL literal carries no value; nothing to store.
    }

    @Override
    public Object parse(SourceData sourceData, int rowIndex, Context context) {
        return null;
    }
}

@TransformParser(values = NullValue.class) registers NullValue.class in parserMap, so getTransformParser resolves it through the reflective constructor branch, and NULL literals evaluate to constant null.

Resulting Behavior

• ifnull(null, numeric2) evaluates its first argument to null, so IfNullFunction falls back to the second argument as expected.
• Other functions that receive a NULL literal as an argument (e.g. coalesce, num_nulls, concat_ws) no longer fail during parser construction.

Tests

Added a true-NULL-literal case to TestIfNullFunction:

// case6: ifnull(null, numeric2) -- true NULL literal
transformSql = "select ifnull(null,numeric2) from source";
data = "5|3|3|5";
Assert.assertEquals("result=3", output.get(0));

Verified: Tests run: 1, Failures: 0, Errors: 0, Skipped: 0.

Backward Compatibility

• Fully backward compatible.
• No public API or method-signature change; only a new parser class is added.
• Existing column / function / literal parsers are unaffected.

Files Changed

• inlong-sdk/transform-sdk/src/main/java/org/apache/inlong/sdk/transform/process/parser/NullValueParser.java (new)
• inlong-sdk/transform-sdk/src/test/java/org/apache/inlong/sdk/transform/process/function/condition/TestIfNullFunction.java

Verifying this change

(Please pick either of the following options)

• This change is a trivial rework/code cleanup without any test coverage.

• This change is already covered by existing tests, such as:
  (please describe tests)

• This change added tests and can be verified as follows:

  (example:)

  • Added integration tests for end-to-end deployment with large payloads (10MB)
  • Extended integration test for recovery after broker failure

Documentation

• Does this pull request introduce a new feature? (yes / no)
• If yes, how is the feature documented? (not applicable / docs / JavaDocs / not documented)
• If a feature is not applicable for documentation, explain why?
• If a feature is not documented yet in this PR, please create a follow-up issue for adding the documentation