fix: say when a query has nothing to look up

[vshulcz]

before: deja p        → no matches in 205 indexed sessions — try fewer words or --re (query "p")
        deja "a b c"  → same
after:  deja p        → nothing to search for in "p" — every word in it is too short to look up

Neighbours were already handled: two characters get the close-spelling ladder (pu -> pump), an empty argument gets query required. The message states what happened rather than the rule, because the cut is on bytes — л and 舵 are long enough while p is not, and both still search. Closes #828.