Never leave programmers menu empty

When there are no programmers available for the current board, the
programmers menu would remain empty, which would prevent it from
unfolding and could make users think there was something wrong with the
menu.

Now, a disabled item with a message is added if no programmers are
available, which should make it more clear what is going on.

This is a followup for #9373.

Author: matthijskooijman

Diff for: app/src/processing/app/Base.java

@@ -1696,6 +1696,12 @@ public void rebuildProgrammerMenu() {
     addProgrammersForPlatform(boardPlatform, programmerMenus, group);
     if (corePlatform != null)
       addProgrammersForPlatform(corePlatform, programmerMenus, group);
+
+    if (programmerMenus.isEmpty()) {
+      JMenuItem item = new JMenuItem(tr("No programmers available for this board"));
+      item.setEnabled(false);
+      programmerMenus.add(item);
+    }
   }
 
   public void addProgrammersForPlatform(TargetPlatform platform, List<JMenuItem> menus, ButtonGroup group) {