fix: changes empty mp parts error on CompleteMultipartUpload

Fixes #1328

If `CompleteMultipartUpload` is attempted with empty `Parts` list, the gateway used to return `InvalidRequest`. Now it's changed to `MalformedXML`.

s3api/controllers/object-post.go

@@ -278,7 +278,7 @@ func (c S3ApiController) CompleteMultipartUpload(ctx *fiber.Ctx) (*Response, err
 			MetaOpts: &MetaOptions{
 				BucketOwner: parsedAcl.Owner,
 			},
-		}, s3err.GetAPIError(s3err.ErrEmptyParts)
+		}, s3err.GetAPIError(s3err.ErrMalformedXML)
 	}
 
 	var mpuObjectSize *int64

s3api/controllers/object-post_test.go

@@ -404,7 +404,7 @@ func TestS3ApiController_CompleteMultipartUpload(t *testing.T) {
 						BucketOwner: "root",
 					},
 				},
-				err: s3err.GetAPIError(s3err.ErrEmptyParts),
+				err: s3err.GetAPIError(s3err.ErrMalformedXML),
 			},
 		},
 		{