AccessKit Visible Alt Text: Fix empty captions on GIFs without alt text

[marcustyphoon]

Description

Simple enough.

This skips processing image elements with alt attributes that are actually empty strings, treating them the same way we do images without the attribute at all. This fixes empty caption elements being added below GIFs without alt text.

Resolves #1687.

Testing steps

• Confirm that GIFs in posts that don't have alt text (try https://www.tumblr.com/search/gif) aren't affected by AccessKit Visible Alt Text.
• Confirm that images with alt text (try https://www.tumblr.com/search/described) are still affected by AccessKit Visible Alt Text.
• Confirm that GIFs with alt text (try https://www.tumblr.com/meltknuckles/757727655146799104/a-herd-of-dairy-cow-isopods-grazing-the-great) are still affected by AccessKit Visible Alt Text.