Avoid hard-erroring out of the UI when trying to view an empty query

[no-identd]

Opening the UI with ui empty and then (with nothing entered) hitting M-v closes the UI and yields:

*** Exception: /home/[REDACTED]/digraphs-with-text/src/Dwt/UI/State.hs:(101,7)-(102,64): Irrefutable pattern failed for pattern commandString : _

That seems unnecessary, especially since it can easily lead to data loss (unless you immediately make use appropriate use of it in the next evaluation, e.g. by going ui it), as this happens any time the top window lacks content.