fix: clear stale tag relations when updating an API with empty tags (#6518)

[yykaue]

What is this PR for?

Fixes #6518.

This PR fixes stale API tag relations when an API is updated with an explicit empty tagIds list.

Changes

• Delete existing tag relations when tagIds is an empty list.
• Avoid invoking batch insertion for an empty list.
• Preserve the existing behavior where null means the API tag relations should not be changed.
• Add regression tests covering both empty and null tag lists.

Verification

• ApiServiceTest: 8 tests passed.
• Checkstyle passed with no violations.

@Aias00 Could you please help review this PR? Thank you!

[Aias00]

PR #6948 Review — fix: clear stale tag relations when updating an API with empty tags (#6518)

Verdict: APPROVE

Scope

shenyu-admin ApiServiceImpl.update: fix stale tag_relation rows when an API is updated with
an explicit empty tagIds list.

Verification

• Bug: original if (CollectionUtils.isNotEmpty(apiDTO.getTagIds())) skipped the whole block for
  an empty list, leaving stale tag relations untouched.
• Fix: guard on Objects.nonNull(apiDTO.getTagIds()) (head file L24 imports java.util.Objects)
  → always deleteByApiId(...); then re-insert only when non-empty. Semantics: null = leave
  relations unchanged (preserved), empty list = clear, non-empty = replace. Exactly the intended
  behavior.
• tagRelationMapper.deleteByApiId/batchInsert are existing methods (unchanged).
• Tests added: testUpdateWithEmptyTagIdsClearsRelations asserts deleteByApiId invoked and
  batchInsert never; testUpdateWithNullTagIdsKeepsRelations asserts neither called.

Notes

• Correct, minimal bug fix with solid regression coverage. Approving.