[Fix rubocop#762] Fix an error for Rails/FreezeTime

Fixes rubocop#762.

This PR fixes an error for `Rails/FreezeTime` when using `travel_to` with
an argument of `current` method without receiver.

changelog/fix_an_error_for_rails_freeze_time.md

@@ -0,0 +1 @@
+* [#762](https://github.com/rubocop/rubocop-rails/issues/762): Fix an error for `Rails/FreezeTime` when using `travel_to` with an argument of `current` method without receiver. ([@koic][])

lib/rubocop/cop/rails/freeze_time.rb

@@ -44,6 +44,7 @@ class FreezeTime < Base
 
         def on_send(node)
           child_node, method_name = *node.first_argument.children
+          return unless child_node
           return unless current_time?(child_node, method_name) || current_time_with_convert?(child_node, method_name)
 
           add_offense(node) { |corrector| corrector.replace(node, 'freeze_time') }

spec/rubocop/cop/rails/freeze_time_spec.rb

@@ -70,4 +70,10 @@
       travel_to(Time.zone.yesterday.in_time_zone)
     RUBY
   end
+
+  it 'does not register an offense when using `travel_to` with an argument of `current` method without receiver' do
+    expect_no_offenses(<<~RUBY)
+      travel_to(current)
+    RUBY
+  end
 end