Do not retry plan availability check when there are no changes in the changeset

[eatwithforks]

We had an empty changeset and miasma retries 10 times which does not make any sense because an empty changeset will never be available.

miasma should read the status response and check if its "The submitted information didn't contain changes. Submit different information to create a change set." and then fail fast.

would you accept a PR to fix it?
@chrisroberts