avoid comparison error in dictdb hub history

in rare situations, display data can arrive at the Hub before the execution request, in which case `submitted` is undefined.

should address #2821, but I need to see Jenkins succeed,
since that is the only place this comes up.
Merge pull request #2829 from minrk/nodate

avoid comparison error in dictdb hub history

in rare situations, display data can arrive at the Hub before the execution
request, in which case `submitted` is undefined.

should address #2821, but I need to see Jenkins succeed, since that is the
only place this comes up.