Add graphql endpoint to return blank branch rules when branch rules don't exist

See Handle "All branches" and "All protected branches" (#388129 - closed) for details. This issue tracks the BE work necessary to close out the linked issue

The new graphql node accepts 2 arguments

id: BranchRuleID
buildMissing: Boolean

passing buildMissing: true will return Projects::AllBranchesRule and Projects::AllProtectedBranchesRule objects even if they don't have any persisted rules yet.