allow matching on files whose content is not present

allow matching on files whose content is not present

Anything that needs to examine the file content will fail to match,
or fall back to other available information. But the intent is that the
matcher be checked for matchNeedsFileContent and only be used if it does
not, so the exact behavior doesn't much matter as it should never
happen.

The real point of this is to not need to provide a dummy content file
when matching.

This commit was sponsored by Martin D on Patreon.