compose: Clear stream id when prompting user for stream selection.

Now we clear the stream id from the compose state on opening the stream
selection dropdown when no stream is specified.

This fixes the bug where the compose box placeholder text would refer to
the previously selected stream, even when the open stream dropdown had
no stream selected.

web/src/compose_actions.js

@@ -251,6 +251,7 @@ export function start(msg_type, opts) {
         compose_recipient.on_compose_select_recipient_update();
     } else {
         // Open stream selection dropdown if no stream is selected.
+        compose_state.set_stream_id("");
         compose_recipient.open_compose_recipient_dropdown();
     }
     compose_state.topic(opts.topic);