update tfk8s tests

[ljluestc]

This PR addresses the panic: value is null crash when processing the IstioOperator manifest (issue #62) by adding the stripNullFields function to remove null values from YAML manifests. It also fixes failing tests and adds validation for the IstioOperator case.

Changes:

• Added stripNullFields function to recursively remove null fields when --strip-null is used.
• Updated TestYAMLToHCLStripNull to use four-space indentation, matching terraform.FormatValue output.
• Fixed TestYAMLToTerraformResourcesMapOnly to set mapOnly=true and updated expected HCL.
• Added TestYAMLToHCLIstioOperatorStripNull to verify the IstioOperator manifest is processed correctly.
• Ensured all tests pass with make test.

Testing:

• Ran make test to confirm all tests pass.

[jrhouston]

Thanks for adding this @ljluestc I pushed a commit removing some redundant logic and adding an additional case to the test. I see you've left this in draft, was there something else you were going to add here? If not I'm happy to merge it.