add count validation in bulk create nodes use case

app/Node/UseCases/BulkCreateNodes.php

@@ -39,6 +39,10 @@ class BulkCreateNodes
             throw new BadRequestException('count is required');
         }
 
+        if ($request->count < 1) {
+            throw new BadRequestException('count must be at least 1');
+        }
+
         $text = $this->textRepo->find($request->textId);
         if ($text === null) {
             throw new DomainException("Text with id: {$request->textId} doesnt exist");