Fix typing notice not sent when composer cleared

Make sure to send a "we're not typing anymore" notice to the server when
the composer is cleared, e.g. when sending a message.

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

@@ -103,7 +103,7 @@ HTextArea {
 
     // TODO: make this more declarative
     onTextChanged: {
-        if (utils.isEmptyObject(usableAliases)) {
+        if (! text || utils.isEmptyObject(usableAliases)) {
             writingUserId = Qt.binding(() => chat.userId)
             toSend        = text
             setTyping(Boolean(text))