fix: allow empty patches files

[ralgozino]

Hello!

Disclaimer: this is my first PR to Kustomize, even though I tried to follow the guidelines, please let me know if I missed anything / should change something.

This PR fixes the issue that I opened and triaged #5956: tl;dr pointing to empty patches files resulted in an error instead of ignoring the files.

As per guidelines, I first added a test that replicates the issue: TestEmptyPatchFilesShouldBeIgnored

Then the fix is pretty straightforward, call transformJson6902 only if there are json patches to apply.

Note: I also added a test to validate that patchesStrategicMerge still behaves the same and that I did not introduced a regression on a deprecated feature. Please let me know if this is ok.

Thanks for kustomize and for your time in reviewing this PR :-)

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[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: ralgozino
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