GH-47447: [C++] Fix replace_with_mask for null type arrays

[AnkitAhlawat7742]

Fix #47447

Rationale for this change

replace_with_mask crashes when called with null type arrays because a code path incorrectly returns Status::OK() from a function with return type Result<int64_t>.
This PR fixes the issue by ensuring the function returns a valid int64_t value instead of Status::OK() for successful executon

What changes are included in this PR?

1. Fixed the replace_with_mask kernel implementation for null type arrays.
2. Replaced the invalid Status::OK() return path with a valid int64_t result.
3. Prevented construction of Result<int64_t> with an OK Status.

Are these changes tested?

Yes, manually run the test case

Are there any user-facing changes?

No

This PR contains a "Critical Fix".

This change fixes a crash in replace_with_mask when operating on null type arrays. The crash occurs due to an invalid successful return path internally constructing Result with Status::OK().

• GitHub Issue: [C++][Python] replace_with_mask crashes when null type inputs are used #47447

[github-actions]

⚠️ GitHub issue #47447 has been automatically assigned in GitHub to PR creator.