Fix final null array refs in kernels

[patrickmaub]

Fixes #92.

Summary:

• Detects final array fields that are null on the kernel instance.
• Emits those fields as literal NULL pointers in generated OpenCL and omits the corresponding host-side kernel argument.
• Skips final-null array fields when building JNI KernelArg metadata.
• Enables the existing NullRefTest as an OpenCL-only regression and adds a codegen regression for the generated signature/body.

Testing:

• Manual Java 8 compile with javac on JDK 21.
• JUnitCore: com.aparapi.codegen.test.NullCheckTest, com.aparapi.codegen.test.FinalNullArrayTest, com.aparapi.runtime.NullRefTest.

Local note: the runtime OpenCL test is skipped on my machine because the Aparapi/OpenCL native library is not loadable here; the codegen regression verifies the null array is no longer passed through JNI and is initialized as NULL in the kernel.

This PR is intended to claim the bounty on #92. Payout details can be provided if accepted.