Fix updating value stream record without stages

What does this MR do?

This MR fixes a bug when updating value streams with no associated stages. This can happen if the value stream was created before 13.9.

How to reproduce it:

1. Make sure you're on ultimate
2. Go to a group and record the id
3. In rails console create a value stream: Group.find(id).value_streams.create(name: 'test')
4. Go to the Group -> Analytics -> Value Stream page
5. Select the test value stream on the top right dropdown and click edit
6. Add a new stage on the bottom and try to save it.

Screenshots (strongly suggested)

Video: https://gitlab.slack.com/files/UH62Q221E/F01TKJBKJTZ/value_stream.mp4

Thanks @mvanremmerden for reporting it!

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