phpstan fixes

Author: GromNaN

src/Eloquent/Builder.php

@@ -11,6 +11,7 @@
 use MongoDB\Laravel\Collection;
 use MongoDB\Laravel\Helpers\QueriesRelationships;
 use MongoDB\Laravel\Internal\FindAndModifyCommandSubscriber;
+use MongoDB\Laravel\Query\AggregationBuilder;
 use MongoDB\Model\BSONDocument;
 use MongoDB\Operation\FindOneAndUpdate;
 
@@ -57,15 +58,15 @@ class Builder extends EloquentBuilder
     ];
 
     /**
-     * @return ($function === null ? PipelineBuilder : self)
+     * @return ($function is null ? AggregationBuilder : self)
      *
      * @inheritdoc
      */
     public function aggregate($function = null, $columns = ['*'])
     {
         $result = $this->toBase()->aggregate($function, $columns);
 
-        return $result ?? $this;
+        return $result ?: $this;
     }
 
     /** @inheritdoc */

src/Query/Builder.php

@@ -301,7 +301,7 @@ private function getAggregationBuilder(): AggregationBuilder
         // Distinct query
         if ($this->distinct) {
             // Return distinct results directly
-            $column = $columns[0] ?? '_id';
+            $column = $this->columns[0] ?? '_id';
 
             $agg->group(
                 _id: \MongoDB\Builder\Expression::fieldPath($column),
@@ -596,7 +596,7 @@ public function generateCacheKey()
         return md5(serialize(array_values($key)));
     }
 
-    /** @return ($function === null ? PipelineBuilder : self) */
+    /** @return ($function is null ? AggregationBuilder : mixed) */
     public function aggregate($function = null, $columns = [])
     {
         if ($function === null) {