Fix a buffer overflow in INFORMATION_SCHEMA.GLOBAL_VARIABLES, caused by MDEV-12179

pretty_print_engine_list(): When the plugin list is empty, return
an empty string instead of allocating a buffer of 0 bytes and then
trying to write a NUL byte into it.

sql/set_var.cc

@@ -1483,7 +1483,7 @@ pretty_print_engine_list(THD *thd, plugin_ref *list)
   size_t size;
   char *buf, *pos;
 
-  if (!list)
+  if (!list || !*list)
     return thd->strmake("", 0);
 
   size= 0;