FIX: Gracefully handle empty files for imgmath depth extraction

[timhoffm]

Closes #14466.

While empty files are unexpected and likely an error, read_svg_depth() is not the place to flag empty files. It is designed to handle files without depth comment, and that should also be true for the edge case of an empty file.

[jdillard]

LGTM. The only concern I see (and it is a small one), is that the whole SVG is loaded into memory with list(f). To keep the small line buffer you could do something like:

with open(filename, encoding='utf-8') as f:
    line = None
    for line in f:
        pass
if line is not None:
    if matched := depthsvgcomment_re.match(line):
        return int(matched.group(1))
return None