return InvalidRequest when no parts are provided (#17395)

cmd/api-errors.go

@@ -149,6 +149,7 @@ const (
 	ErrMethodNotAllowed
 	ErrInvalidPart
 	ErrInvalidPartOrder
+	ErrMissingPart
 	ErrAuthorizationHeaderMalformed
 	ErrMalformedPOSTRequest
 	ErrPOSTFileRequired
@@ -686,6 +687,11 @@ var errorCodes = errorCodeMap{
 		Description:    "One or more of the specified parts could not be found.  The part may not have been uploaded, or the specified entity tag may not match the part's entity tag.",
 		HTTPStatusCode: http.StatusBadRequest,
 	},
+	ErrMissingPart: {
+		Code:           "InvalidRequest",
+		Description:    "You must specify at least one part",
+		HTTPStatusCode: http.StatusBadRequest,
+	},
 	ErrInvalidPartOrder: {
 		Code:           "InvalidPartOrder",
 		Description:    "The list of parts was not in ascending order. The parts list must be specified in order by part number.",