MDEV-26637: (plugin) ASAN: main.metadata and user_variables.basic MTR failures after MDEV-26572

Get rid of locking "empty" plugin.
This problem present in our tests in case of malformed frm-file, for example.

Author: sanja-byelkin

sql/sql_plugin.cc

@@ -1068,6 +1068,8 @@ plugin_ref plugin_lock_by_name(THD *thd, const LEX_CSTRING *name, int type)
   plugin_ref rc= NULL;
   st_plugin_int *plugin;
   DBUG_ENTER("plugin_lock_by_name");
+  if (!name->length)
+    DBUG_RETURN(NULL);
   mysql_mutex_lock(&LOCK_plugin);
   if ((plugin= plugin_find_internal(name, type)))
     rc= intern_plugin_lock(lex, plugin_int_to_ref(plugin));