MaterializedViewSupervisorSpec should not allow empty baseDataSource

[chengchengpei]

similar to #5903. baseDataSource should not be empty string.

I am new to druid, but would like to contribute to druid. I am working on this: #6075

[github-actions]

This issue has been marked as stale due to 280 days of inactivity.
It will be closed in 4 weeks if no further activity occurs. If this issue is still
relevant, please simply write any comment. Even if closed, you can still revive the
issue at any time or discuss it on the dev@druid.apache.org list.
Thank you for your contributions.

[github-actions]

This issue has been closed due to lack of activity. If you think that
is incorrect, or the issue requires additional review, you can revive the issue at
any time.