Don't panic if the stageq here is empty; just fall through with NULL
pointers and leave the stage queue flush routine to just do nothing
(since both head and tail here will be NULL.)

This should quieten the "stageq empty" panic where the stageq itself
is empty, but it won't fix the second KASSERT() here "staging queue empty"
as that's likely a different underlying problem.

PR: kern/174283