Remove OpenStruct from primer_octicon cop

OpenStruct will not be included in modern Ruby versions, so we should not rely on it being there.
This removes usage and instead builds the same class instance that is returned above.

irb(main):003> h = RuboCop::AST::HashNode.new(hash)
=> s(:hash)
irb(main):004> h.keys
=> []
irb(main):005> h.pairs
=> []
irb(main):006> h.type
=> :hash

lib/rubocop/cop/primer/primer_octicon.rb

@@ -200,7 +200,7 @@ def string_args_to_string(node)
         def kwargs(node)
           return node.arguments.last if node.arguments.size > 1
 
-          OpenStruct.new(keys: [], pairs: [], type: :hash)
+          RuboCop::AST::HashNode.new("hash")
         end
 
         def icon(node)

test/lib/rubocop/primer_octicon_test.rb

@@ -273,4 +273,13 @@ def test_medium_size
 
     assert_correction "primer_octicon(:icon, size: :medium)"
   end
+
+  def test_no_args
+    investigate(cop, <<-RUBY)
+      octicon
+    RUBY
+
+    raise "run"
+    assert_correction "primer_octicon"
+  end
 end