[Basic] Remove unused inputType parameter in ProjectionDescriptor.createPojoJavaImplementation (#732) - #804

Description

Closes #732

In ProjectionDescriptor.java, the private static method createPojoJavaImplementation declared an unused parameter final BasicDataUnitType<Input> inputType. Unlike createRecordJavaImplementation (which requires RecordType to map field names to indices), POJO projections resolve fields via reflection on the POJO object at runtime and only require the field name.

Changes

• Removed the unused inputType parameter from createPojoJavaImplementation(final String[] fieldNames) and updated its single call site in ProjectionDescriptor.
• Marked Field field as transient in PojoImplementation to prevent NotSerializableException when the SerializableFunction is serialized across tasks/JVMs after execution.
• Added a null check in PojoImplementation.apply to return null if the input object is null.
• Expanded ProjectionDescriptorTest with test cases for null inputs, multi-field error handling, non-existent field error handling, and serialization round-trips.

Type of Change

• Bug fix / Code cleanup (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update

How Has This Been Tested?

• Ran all tests in ProjectionDescriptorTest (5/5 passed).
• Ran all module unit tests in wayang-basic (19/19 passed).
• Verified compilation of downstream dependent modules (wayang-spark).