Fix handling of empty multipart uploads for GCS (#5590)

* Fix handling of empty multipart uploads for GCS

* Clippy

Author: tustvold

object_store/src/gcp/client.rs

@@ -430,6 +430,13 @@ impl GoogleCloudStorageClient {
         multipart_id: &MultipartId,
         completed_parts: Vec<PartId>,
     ) -> Result<PutResult> {
+        if completed_parts.is_empty() {
+            // GCS doesn't allow empty multipart uploads
+            let result = self.put_request(path, Default::default()).send().await?;
+            self.multipart_cleanup(path, multipart_id).await?;
+            return Ok(result);
+        }
+
         let upload_id = multipart_id.clone();
         let url = self.object_url(path);