Robust on raw perl value instead of register value

[dginev]

This was encountered in arXiv:2308.06254v1, where a use of \fam was returning a perl 0 scalar value from readRegisterValue.

I think we should harden readRegisterValue to always return an object, catching these cases and defaulting them to what the coercing would do, or Number if nothing else is known.

With the change in the PR the article converts without a Fatal, instead emitting a warning:

Warning:malformed:register_value Register \fam did not return a proper value, got 0

That can also be made into an error if preferred. Encountered in an old issue when processed with the latest latexml worker, at arXiv/html_feedback#22

Diff without whitespace lints at https://github.com/brucemiller/LaTeXML/pull/2693/files?w=1