Make membership required for create_user_by_membership

spec/support/matchers/access_matchers_for_controller.rb

@@ -25,6 +25,8 @@ module AccessMatchersForController
       user = role
       sign_in(user)
     when *Gitlab::Access.sym_options_with_owner.keys # owner, master, developer, reporter, guest
+      raise ArgumentError, "cannot emulate #{role} without membership parent" unless membership
+
       user = create_user_by_membership(role, membership)
       sign_in(user)
     else
@@ -34,9 +36,7 @@ module AccessMatchersForController
     user
   end
 
-  def create_user_by_membership(role, membership = nil)
-    raise ArgumentError, "cannot emulate #{role} without membership parent" unless membership
-
+  def create_user_by_membership(role, membership)
     if role == :owner && membership.owner
       user = membership.owner
     else