Handle `milestones: nil` in `Release::CreateService#execute`

Nathan Friendrequested to merge
nfriend-fix-release-create-service-when-milestones-nil into master

What does this MR do?

Fixes an issue that caused Release::CreateService#execute to incorrectly assign all project milestones to a newly-created release when #execute was passed a Hash with milestones: nil.

Closes #277396 (closed)

Edited by Nathan Friend

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