fix: ignore empty trailing pieces in sentence count

[Chessing234]

Summary

• Trailing terminators left an empty split, inflating the sentence count.
• Drop empty pieces after the split.

Test plan

• TestVerifySentenceConstraint

GPU_TESTS=bypass
CHANGELOG=

[gemini-code-assist]

Code Review

This pull request updates the verify_sentence_constraint function in open_instruct/if_functions.py to filter out empty or whitespace-only strings resulting from splitting sentences with trailing terminators. Additionally, a new test suite TestVerifySentenceConstraint has been added to open_instruct/test_if_functions.py to validate this behavior, and the CHANGELOG.md has been updated accordingly. There are no review comments, so there is no feedback to provide.

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