Removed incorrect nullable return type from RecordCollection::findFirstCollection.

Author: Bilge

src/Collection/RecordCollection.php

@@ -43,7 +43,7 @@ public function getPreviousCollection(): ?self
         return $this->previousCollection;
     }
 
-    public function findFirstCollection(): ?self
+    public function findFirstCollection(): self
     {
         do {
             $previous = $nextPrevious ?? $this->getPreviousCollection();