"Phabricator is a public place. Describe the project in a way that allows anybody, also outside of the community, to clearly understand what the project is about. Add links to the relevant project pages so anybody can find more information."
That's currently not always the case.
Thus consider custom code which shows a reminder/error when trying to create a project with an empty description, similar to the error shown when not entering a project name.
Could apply this only to new parent projects, and not to subprojects and milestones which might be already "explained" by their parent project's description.
May lead to project descriptions like . or of course. But in that case there's a reason to discuss our current guidelines.