Handle nil attributes by making them untyped

lib/sord/generator.rb

@@ -419,6 +419,9 @@ def add_attributes(item)
           if yard_types.empty?
             Logging.omit("no YARD type given for #{name.inspect}, using untyped", reader || writer)
             parlour_type = Parlour::Types::Untyped.new
+          elsif yard_types.all? { |x| x == 'nil' }
+            # Nil attributes are extremely unusual, so just use untyped
+            parlour_type = Parlour::Types::Untyped.new
           else
             parlour_type = TypeConverter.yard_to_parlour(
               yard_types, reader || writer, @replace_errors_with_untyped, @replace_unresolved_with_untyped)

spec/generator_spec.rb

@@ -1629,4 +1629,27 @@ def foo: () -> void
       end
     RUBY
   end
+
+  it 'handles nil attributes' do
+    YARD.parse_string(<<-RUBY)
+      class A
+        # @return [nil]
+        attr_accessor :x
+      end
+    RUBY
+
+    expect(rbi_gen.generate.strip).to eq fix_heredoc(<<-RUBY)
+      # typed: strong
+      class A
+        sig { returns(T.untyped) }
+        attr_accessor :x
+      end
+    RUBY
+
+    expect(rbs_gen.generate.strip).to eq fix_heredoc(<<-RUBY)
+      class A
+        attr_accessor x: untyped
+      end
+    RUBY
+  end
 end