Ensure none constraint scopes don’t trigger a query

rails#47800 refactored how `Relation#none` worked, and one side effect of this change was that if you had an association like this:

`has_many :comments_none, ->(post) { none }, class_name: "Comment”`

reading the association caused a query to happen, despite the `none` in the passed in scope.

As it turns out, this is because the association performs a merge of the two scopes, and the `@none` instance variable wasn’t getting copied over to the merged scope, so the upfront guard clause checking `@none` in `#exec_main_query`.

Updating the `Merger` object to add a check for null relations and call `none!` ensures the query doesn’t run in this particular scenario.

Author: aharpole

activerecord/lib/active_record/relation/merger.rb

@@ -69,6 +69,8 @@ def merge
           end
         end
 
+        relation.none! if other.null_relation?
+
         merge_select_values
         merge_multi_values
         merge_single_values

activerecord/test/cases/null_relation_test.rb

@@ -59,6 +59,16 @@ def test_null_relation_content_size_methods
     end
   end
 
+  def test_null_relation_used_with_constraints
+    post = Post.first
+    assert_no_queries do
+      scope = post.comments
+      none = Post.none
+      scope = scope.merge(none)
+      assert_equal 0, scope.size
+    end
+  end
+
   def test_null_relation_metadata_methods
     assert_includes Developer.none.to_sql, " WHERE (1=0)"
     assert_equal({}, Developer.none.where_values_hash)