getFontHeight() does not accept null

Some C API functions appear to use the default font when a null LCDFont* is passed in (like getTextWidth()), but getFontHeight() crashes when handed null.

Thanks for catching that! :pray: We have an issue filed to validate all of the C API arguments so we don’t e.g. crash when NULL is passed it, but it looks like we also need to make sure behavior like this is consistent. I’ll make a note!