fix: Omit empty driveId in dir response JSON (#4694)

  When making requests to the files API for directories (e.g to create a
  directory), we always return a `driveId` attribute in the response
  JSON even if the directory is not part of a shared drive while we omit
  it for files.

  We will now omit this attribute when empty for directories as well.

Author: taratatach

web/files/paginated.go

@@ -38,7 +38,7 @@ type dir struct {
 
 type dirJSON struct {
 	*vfs.DirDoc
-	DriveId string `json:"driveId"`
+	DriveId string `json:"driveId,omitempty"`
 }
 
 type file struct {