fix html: Don't fail if a tag does not have some attributes.

[thet]

Fixes:

Traceback (most recent call last):
  File "/home/thet/.buildout/eggs/v5/collective.exportimport-1.15-py3.13-linux-x86_64.egg/collective/exportimport/fix_html.py", line 356, in fix_html_in_content_fields
    clean_text = fixer(clean_text, obj)
  File "/home/thet/.buildout/eggs/v5/collective.exportimport-1.15-py3.13-linux-x86_64.egg/collective/exportimport/fix_html.py", line 525, in img_variant_fixer
    classes = tag["class"]
              ~~~^^^^^^^^^
  File "/home/thet/.buildout/eggs/v5/beautifulsoup4-4.14.3-py3.13-linux-x86_64.egg/bs4/element.py", line 2401, in __getitem__
    return self.attrs[key]
           ~~~~~~~~~~^^^^^