Update login-ldap.php

In some rare occasions, the ldap setup can return null when reading the $attributes[$group_indentifier].

By checking that it isn't empty before shifting the array, we prevent the following error:

* array_shift expected parameter to be array, null given

login-ldap.php

@@ -181,6 +181,10 @@ class LoginLDAPPlugin extends Plugin
 
                     foreach ($groups as $group) {
                         $attributes = $group->getAttributes();
+                        
+                        // make sure we have an array to read
+                        if ( !empty($attributes) && !empty($attributes[$group_indentifier]) && is_array($attributes[$group_indentifier]) )
+                        {
                             $user_group = array_shift($attributes[$group_indentifier]);
                             $user_groups[] = $user_group;
 
@@ -189,6 +193,7 @@ class LoginLDAPPlugin extends Plugin
                             }
                         }
                     }
+                }
 
                 // Merge the LDAP user data with Grav user
                 $grav_user->merge($userdata);