Route migrate process plugin shouldn't assume that the $options variable is always an array

Closes #3156083

core/modules/migrate/src/Plugin/migrate/process/Route.php

@@ -131,7 +131,9 @@ public function transform($value, MigrateExecutableInterface $migrate_executable
           $options['query'] = $route['options']['query'] + $old_query;
           unset($route['options']['query']);
         }
-        $route['options'] = $route['options'] + $options;
+        if (is_array($options)) {
+          $route['options'] += $options;
+        }
         $route['url'] = NULL;
       }
     }