Fix 500 error when genomicDataFilters is empty (#11206)

[heathdutton]

Fix #11206

Describe changes proposed in this pull request:

• Add null/empty validation for genomicDataFilters in fetchGenomicDataCounts and fetchMutationDataCounts endpoints
• Return empty list instead of propagating IndexOutOfBoundsException from MyBatis mapper
• Add unit tests covering empty and null filter scenarios

Checks

• The commit log is comprehensible. It follows 7 rules of great commit messages. We can fix this during merge by using a squash+merge if necessary
• Has tests or has a separate issue that describes the types of test that should be created. If no test is included it should explicitly be mentioned in the PR why there is no test.
• Is this PR adding logic based on one or more clinical attributes? If yes, please make sure validation for this attribute is also present in the data validation / data loading layers (in backend repo) and documented in File-Formats Clinical data section!
• Make sure your PR has one of the labels defined in https://github.com/cBioPortal/cbioportal/blob/master/.github/release-drafter.yml

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@alisman @inodb

[onursumer]

Might be better to use the ok method here as well for consistency.

Suggested change

	return new ResponseEntity<>(Collections.emptyList(), HttpStatus.OK);
	return ResponseEntity.ok(Collections.emptyList());

[onursumer]

we probably don't need this line of comment with issue reference. same applies to the other tests below.