### What problem does this PR solve?
Fixed 12787 with syntax error in generated MySql json path expression
https://github.com/infiniflow/ragflow/issues/12787
### Type of change
- [x] Bug Fix (non-breaking change which fixes an issue)
#### What was fixed:
- Changed line 237 in ob_conn.py from value_str = get_value_str(value)
if value else "" to value_str = get_value_str(value)
- This fixes the bug where falsy but valid values (0, False, "", [], {})
were being converted to empty strings, causing invalid SQL syntax
#### What was tested:
- Comprehensive unit tests covering all edge cases
- Regression tests specifically for the bug scenario
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Co-authored-by: Kevin Hu <kevinhu.sh@gmail.com>
