<prefix>_KEYWORDS_MISSING_VALUES does not contain entries for keywords missing at the call site
When function arguments are parsed using cmake_parse_arguments, an element is added to <prefix>_KEYWORDS_MISSING_VALUES for all keywords that were provided at the call site, without a following value.
If the keyword is not provided at all, the keyword is not added to KEYWORDS_MISSING_VALUES. I think the correct behaviour would be to handle both cases in the same way.