fix: ValueTracker.evaluate() should not skip falsy values like 0

[KooshaPari]

Bug

ValueTracker.evaluate() used if not value to skip empty/None values, but this also skipped falsy values like 0, False, and empty strings "". This caused incorrect type detection for columns containing only falsy values - for example, a column of all empty strings would be guessed as integer instead of text.

Fix

Changed if not value or not self.couldbe to if value is None or not self.couldbe so that only None is skipped, while falsy values like 0, False, and "" are properly evaluated.

Testing

• Added test_value_tracker_evaluates_falsy_zero - verifies zeros narrow to integer
• Added test_value_tracker_evaluates_falsy_empty_string - verifies empty strings narrow to text
• Added test_value_tracker_evaluates_falsy_false - verifies False narrows to integer
• All 22 tests in test_utils.py pass

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📚 Documentation preview 📚: https://sqlite-utils--863.org.readthedocs.build/en/863/

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