When a user draws annotation boxes to mark OCR regions, the blocks are
created with source=MANUAL and empty text. upsertGuidedBlock was
protecting all MANUAL blocks unconditionally, so guided OCR silently
produced no output for these drawn-but-empty blocks.
Changed the guard to only protect non-empty MANUAL blocks — empty ones
are treated like OCR blocks and get their text filled in.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>