Use empty hotkey button when key is unbound across all versions

This happened to already be the case in 1.16.3 (because of another bug), this
commit makes it intentional and consistent across all versions.
Using the empty label over NONE or another label because it doesn't change the
width of the button and therefore fits nicer with all the other buttons.

src/main/java/com/replaymod/core/KeyBindingRegistry.java

@@ -185,6 +185,14 @@ public class KeyBindingRegistry extends EventRegistrations {
             return MCVer.getBoundKey(keyBinding);
         }
 
+        public boolean isBound() {
+            //#if MC>=11400
+            return !keyBinding.isUnbound();
+            //#else
+            //$$ return keyBinding.getKeyCode() != 0;
+            //#endif
+        }
+
         public void trigger() {
             KeyBindingAccessor acc = (KeyBindingAccessor) keyBinding;
             acc.setPressTime(acc.getPressTime() + 1);