Use IsNullOrWhiteSpace to check file upload id is present

Author: KoditkarVedant

Src/Notion.Client/Api/FileUploads/Send/FileUploadsClient.cs

@@ -15,7 +15,7 @@ public async Task<SendFileUploadResponse> SendAsync(
                 throw new ArgumentNullException(nameof(sendFileUploadRequest));
             }
 
-            if (string.IsNullOrEmpty(sendFileUploadRequest.FileUploadId))
+            if (string.IsNullOrWhiteSpace(sendFileUploadRequest.FileUploadId))
             {
                 throw new ArgumentNullException(nameof(sendFileUploadRequest.FileUploadId));
             }