fix: guard empty fixed-width vectors in CBloomFilter to avoid index-out-of-range panic

[SAY-5]

What type of PR is this?

• API-change
• BUG
• Improvement
• Documentation
• Feature
• Test and CI
• Code Refactoring

Which issue(s) this PR fixes:

issue #25618

What this PR does / why we need it:

The three fixed-width CBloomFilter methods (testAndAddFixedVector, testFixedVector, addFixedVector) take &fixedData[0] and &results[0] unconditionally, which panics with index out of range [0] with length 0 when the input vector is empty. Their varlena counterparts (testAndAddVarlenaVector, testVarlenaVector, addVarlenaVector) already return early on v.Length() == 0, so behavior on empty input was inconsistent by type.

This adds the same zero-length early return to the fixed-width path so TestVector/AddVector/TestAndAddVector are no-ops on an empty vector instead of panicking, matching the varlena behavior. A regression test covers all three fixed-width APIs.

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[gouhongshen]

Codex automated review

No concrete regressions found. The guards correctly make empty fixed-width vectors no-ops, matching the existing varlena behavior and issue contract. Local focused tests could not run because Go is unavailable; PR CI reported success.