fix: allow to return undefined from avatarUrl callback

https://web.tracklify.com/project/2b7ZVgE5/AdminForth/1196/kqyU8qa9/what-i-should-return-if-avatar

Author: yaroslav8765

adminforth/documentation/docs/tutorial/07-Plugins/05-upload.md

@@ -642,7 +642,7 @@ And finally add this callback:
       //diff-add
       if (adminUser.dbUser.avatar === null || adminUser.dbUser.avatar === undefined || adminUser.dbUser.avatar === '') {
         //diff-add
-        return '';
+        return undefined;
         //diff-add
       }
       //diff-add

adminforth/documentation/docs/tutorial/07-Plugins/11-oauth.md

@@ -566,7 +566,7 @@ And finally add this callback:
       //diff-add
       if (adminUser.dbUser.avatar === null || adminUser.dbUser.avatar === undefined || adminUser.dbUser.avatar === '') {
         //diff-add
-        return '';
+        return undefined;
         //diff-add
       }
       //diff-add

adminforth/types/Back.ts

@@ -1072,7 +1072,7 @@ export interface AdminForthInputConfig {
       /**       
        * Function to return avatar URL for user.
        */
-      avatarUrl?: ((adminUser: AdminUser) => string | Promise<string>)
+      avatarUrl?: ((adminUser: AdminUser) => string | Promise<string> | undefined | Promise<undefined>),
 
       /**
        * Remember me duration for "Remember Me" checkbox on login page.