fix: honor empty-string metadata overrides

[wunianze666-netizen]

Summary

This PR makes metadata overrides in extract_metadata_from_image() honor explicitly provided empty strings instead of falling back to embedded image metadata.

Problem

The helper currently uses override or pil_image.info.get(...) for metadata, workflow, and graph overrides. That means an explicit empty-string override is treated as falsy and ignored, so the function silently falls back to the image's embedded metadata.

Changes

• use is not None checks for metadata/workflow/graph overrides
• add a test to ensure empty-string overrides do not fall back to image metadata

Verification

• python -m py_compile invokeai/app/api/extract_metadata_from_image.py tests/app/test_extract_metadata_from_image.py
• Not run: pytest is not available in this environment, so I could not execute the new test locally.