Fix CP Listing breaking when there's a missing user (#2410)

Co-authored-by: Duncan McClean <duncan@mcclean.co.uk>

src/Fieldtypes/Users.php

@@ -97,13 +97,17 @@ public function preProcessIndex($data)
         }
 
         return $users->map(function ($user) {
+            if (! $user) {
+                return null;
+            }
+
             return [
                 'id' => $user->id(),
                 'title' => $user->get('name', $user->email()),
                 'edit_url' => $user->editUrl(),
                 'published' => null,
             ];
-        });
+        })->filter()->values();
     }
 
     protected function augmentValue($value)