Update storage_management.mdx (supabase#18385)

Handle the case in which the new `avatar_url` is set to null.

Author: ilbertt

apps/docs/components/MDX/storage_management.mdx

@@ -64,7 +64,7 @@ declare
  avatar_name text;
 begin
  if coalesce(old.avatar_url, '') <> ''
- and (tg_op = 'DELETE' or (old.avatar_url <> new.avatar_url)) then
+ and (tg_op = 'DELETE' or (old.avatar_url <> coalesce(new.avatar_url, ''))) then
  -- extract avatar name
  avatar_name := old.avatar_url;
  select