Clarify difference between \problemname and \problemname{}

[niemela]

If I understand LaTeX correctly, (and our spec), \problemname should mean "Insert problem name here", and \problemname{} should mean "set name to empty string (and insert it here)".

Do we want that? An empty name should be an error anyway? Would it be better is \problemname{} was specified to mean \problemname or to be an error?

[RagnarGrootKoerkamp]

Some editors warn on stuff like some sentece with a \command blah blah, encouraging to write \command{} instead. So probably we should make \problemname{} and \problemname equivalent to both use the name given in the yaml.