iproto: trigger input even if there was no output

IPROTO triggers input processing after sending output, provided the
output channel isn't clogged, which totally makes sense. There's one
nuance here: currently, input is triggered only if something was written
to the output channel. Before the IPROTO_WATCH request type was added by
commit 4be5de4c ("iproto: add watchers
support"), every request had a response so it worked just fine. The new
request type is special - the server doesn't reply to it. As a result,
if input was blocked upon reaching the box.cfg.net_msg_max limit, it
might never get resumed, because input processing might never be
triggered. Fix this issue by triggering input processing even if there
was no output to flush.

No user is affected, because IPROTO_WATCH hasn't been officially
released yet so neither changelog nor doc is required.

Closes #6818

NO_CHANGELOG=bug not released
NO_DOC=NO_CHANGELOG