Fix query for RoleImplications with a NULL source context

* SQL's `IN` operator never matches `NULL`, so implications from Role
  assignments in the `null` context were not found and permissions were
  missing (specifically, the implication from ProgramCreator ->
  ObjectEditor was not happening).
Merge pull request #696 from reciprocity/feature/fix-role-implication-query-for-null-contexts

Fix query for RoleImplications with a NULL source context