We now pass empty string as stream name instead of undefined when
passing the narrow operands to make sure we show an invalid narrow
instead of error. We do same thing for drafts.
We also need to add a check to make sure narrow_state.stream_sub
is defined before accessing stream_id field in the code to filter
scheduled messages for a narrow.
These changes adds a new scheduled message indicator in conversation views
which informs the user of the number of messages that are scheduled to be
sent to the current view.
Fixes: #25584.