Add logging to track a Notice messages

- Part of https://github.com/friendica/friendica/issues/10755#issuecomment-933040517

src/Model/User.php

@@ -850,6 +850,10 @@ class User
 	 */
 	public static function getAvatarUrl(array $user, string $size = ''):string
 	{
+		if (empty($user['nickname'])) {
+			DI::logger()->warning('Missing user nickname key', ['trace' => System::callstack(20)]);
+		}
+
 		$url = DI::baseUrl() . '/photo/';
 
 		switch ($size) {