[Fix #559] Fix an error for Rails/RelativeDateConstant

Fixes #559.

This PR fixes an error for `Rails/RelativeDateConstant`
when using multiple assignment.

Author: koic

changelog/fix_error_for_rails_relative_date_constant.md

@@ -0,0 +1 @@
+* [#559](https://github.com/rubocop/rubocop-rails/issues/559): Fix an error for `Rails/RelativeDateConstant` when using multiple assignment. ([@koic][])

lib/rubocop/cop/rails/relative_date_constant.rb

@@ -90,7 +90,7 @@ def offense_range(name, value)
         end
 
         def nested_relative_date(node, &callback)
-          return if node.block_type?
+          return if node.nil? || node.block_type?
 
           node.each_child_node do |child|
             nested_relative_date(child, &callback)

spec/rubocop/cop/rails/relative_date_constant_spec.rb

@@ -172,4 +172,14 @@ class SomeClass
       RUBY
     end
   end
+
+  context 'when using multiple assignment' do
+    it 'does not register an offense' do
+      expect_no_offenses(<<~RUBY)
+        class SomeClass
+          FOO, BAR = *do_something
+        end
+      RUBY
+    end
+  end
 end