fix: Allow #top and the empty string as special-case non-missing IDs

[elliott-with-the-longest-name-on-github]

As noted in #12367, #top is a perfectly valid hashlink for the top of the document. (TODO on rest of description, I'm going to bed.)

Also, AFAICT, there were no tests for handleMissingId in the first place, so I added some.

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[eltigerchino]

Not sure why but the added test is failing.

[eltigerchino]

@elliott-with-the-longest-name-on-github I managed to get the CI tests passing. Was there anything else you wanted to do for this PR besides the changeset?