core: ignore key bindings with empty command

This makes possible to disable a key hiding another (because shorter than the
other, and beginning with the same key).

For example if key "meta-a" is set to an empty command, it would not hide any
more the key "meta-a,1" which has a non-empty command.

ChangeLog.adoc

@@ -15,6 +15,7 @@ For a list of important changes that require manual actions, please look at rele
 
 New features::
 
+  * core: ignore key bindings with empty command
   * core: add support of quotes in commands `/key bind` and `/key bindctxt`
   * core: evaluate command given to `/repeat` with contextual variables (issue #2007)
   * core: add option `callbacks` in command `/debug`

src/gui/gui-key.c

@@ -1653,6 +1653,10 @@ gui_key_search_part (struct t_gui_buffer *buffer, int context,
     for (ptr_key = (buffer) ? buffer->keys : gui_keys[context]; ptr_key;
          ptr_key = ptr_key->next_key)
     {
+        /* ignore keys with no command */
+        if (!ptr_key->command || !ptr_key->command[0])
+            continue;
+
         if (ptr_key->key
             && ((context != GUI_KEY_CONTEXT_CURSOR)
                 || (ptr_key->key[0] != '@')))