[8.x] Fix Eager loading partially nullable morphTo relations (#36129)

* Filter non-empty keys by type

* Fix failed style check

* Remove unnecessary callback

src/Illuminate/Database/Eloquent/Relations/MorphTo.php

@@ -164,7 +164,7 @@ protected function gatherKeysByType($type, $keyType)
                     ? array_keys($this->dictionary[$type])
                     : array_map(function ($modelId) {
                         return (string) $modelId;
-                    }, array_keys($this->dictionary[$type]));
+                    }, array_filter(array_keys($this->dictionary[$type])));
     }
 
     /**