graph/sharedWithMe: Fix 'createdBy' attribute for resources from project spaces

Don't return a meaningless userid anymore for shares created from files or directories
in a project space. These resources don't have a real owner.

Fixes: #8314
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Internal Server Error - Gitea: Git with a cup of tea
500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f