fix: normalize None text to empty string in Text elements

[TrueFurina]

Summary

Fixes #4084: when an Image element is created with text=None (e.g. yolox hi-res model detects a bbox with missing text), str(element) returned a non-string, breaking printing/accessing the element.

Changes

• unstructured/documents/elements.py:
  • Text.__init__: normalize None text to "" so self.text is always a str
  • Text.__str__: fall back to "" when self.text is None (defensive, e.g. deserialized elements)
• test_unstructured/documents/test_elements.py: add test_Image_text_none_str_returns_empty_string covering both image.text == "" and str(image) == ""

Verification

• ast.parse passes on both files
• Local assertions: Image(text=None) → text='' and str=''; Image(text='hello') unchanged → text='hello', str='hello'
• Docs-only class hierarchy: Image(Text), no other behavior changed