Fix: "COMMAND" request doesn't require a response.

Force output to ''.
WaitQuery: Fix: check condition_fulfilled() at first. timeout after.
To stay consistent with tests.
Don't try to send an empty data.
test: Fix wait_query test according to last change.
Cleaned wait_query.condition_fulfilled():

As condition_fulfilled() is only used to check its bool value then
it's better to return True (on non-empty result) or False.
Enh/Fix: don't try to send an empty response.
docstring