fix empty message

Author: Androz2091

src/ch/epfl/chacun/gui/MessageBoardChatUI.java

@@ -34,6 +34,7 @@ public static Node create(Consumer<String> sendMessage, TextMaker textMaker) {
         textField.setPromptText(textMaker.enterChatMessage());
 
         textField.setOnAction(e -> {
+            if (textField.getText().isBlank()) return;
             sendMessage.accept(textField.getText());
             textField.clear();
         });

src/ch/epfl/chacun/net/WSClient.java

@@ -181,6 +181,7 @@ public void sendAction(String message) {
      * @param message the message to send
      */
     public void sendChatMessage(String message) {
+        Preconditions.checkArgument(!message.isEmpty());
         sendText(STR."GAMEMSG.\{java.net.URLEncoder.encode(message, StandardCharsets.UTF_8)}");
     }