resource/gitlab_project: Fix passing `false` to API for explicitly set optional attributes

Timo Furrerrequested to merge
github/fork/timofurrer/feature/issue-1151 into main

I think at one point we have to decide on a consistent strategy about how and when to set optional attributes for the API options ... I have a few ideas and I'll write them up in an issue.

Closes: #1151 (closed)

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