Fix up/down not working with cursor in @word

In the composer, when the text cursor was inside a word starting
with a @ but not matching any username (thus the completion UI being
invisible), pressing up/down arrow would not do anything instead of
going to the prev/next line.

TODO.md

@@ -3,10 +3,7 @@
 - handle invalid access token
 - If an account is gone from the user's config, discard UI state last page
 - room A-Z sorting
-- up/down doesn't work in the middle of a @word for which autocompletion isn't
-  open because no matches
 - filter > enter > room list is always scrolled to top
-- change profile → post in room → message shows as old profile
 - session list: prevent tab-focusing the delegates
 - refresh server list button
 

src/gui/Pages/Chat/Composer/Composer.qml

@@ -42,7 +42,7 @@ Rectangle {
 
             MessageArea {
                 id: messageArea
-                autoCompletionOpen: userCompletion.open
+                autoCompletionOpen: userCompletion.open && userCompletion.count
                 usersCompleted: userCompletion.usersCompleted
 
                 onAutoCompletePrevious: userCompletion.previous()