fix: coerce None text to empty string in Text element

[themavik]

Summary

Fixes #4084

When yolox detects a Picture element with bounding-box coordinates but no OCR text, the text attribute is set to None. When this element is normalized to an Image (which extends Text), the None value is preserved. Subsequent calls to str(element) then return None (the Python object, not the string "None"), violating the type contract (text: str) and causing errors in downstream processing.

Changes

In Text.__init__, coerce None to the empty string:

# Before
self.text: str = text

# After
self.text: str = text if text is not None else ""

This matches the existing defensive pattern in Element.__init__ which sets self.text = self.text if hasattr(self, "text") else "", and ensures __str__ always returns a valid string.

Test plan

• Create an Image element with text=None and verify str(element) returns "" instead of None
• Run partition_image with the yolox model on images that produce Picture annotations without text
• Verify all existing tests pass