Fill my intentionally empty block for RuboCop

An explicit nil tests just as well while satisfying the linter.

test/components/clipboard_copy_test.rb

@@ -17,6 +17,7 @@ def test_renders_simple
   def test_requires_aria_label_when_empty
     assert_raises(ArgumentError) do
       render_inline Primer::ClipboardCopy.new(value: "my-branch-name") do
+        nil
       end
     end
   end