don't clear blank messages that are not send

src/main/kotlin/org/briarproject/briar/desktop/conversation/ConversationViewModel.kt

@@ -83,11 +83,12 @@ constructor(
     fun sendMessage() {
         try {
             val text = _newMessage.value
-            _newMessage.value = ""
 
             // don't send empty or blank messages
             if (text.isBlank()) return
 
+            _newMessage.value = ""
+
             val start = LogUtils.now()
             val m = createMessage(text)
             messagingManager.addLocalMessage(m)