Fix Rails/FilePath cop error on join method with implicit receiver

```console
echo 'join(Rails.root, path)' | rubocop --only Rails/FilePath  --stdin bug.rb -d

n error occurred while Rails/FilePath cop was inspecting bug.rb:1:0.
undefined method `each_node' for nil
lib/rubocop/cop/rails/file_path.rb:54:in `rails_root_nodes?'
lib/rubocop/cop/rails/file_path.rb:60:in `rails_root_join_nodes?'
lib/rubocop/cop/rails/file_path.rb:108:in `check_for_rails_root_join_with_string_arguments'
lib/rubocop/cop/rails/file_path.rb:70:in `on_send'
```

changelog/fix_merge_pull_request_1392_from.md

@@ -0,0 +1 @@
+* [#1397](https://github.com/rubocop/rubocop-rails/pull/1397): Fix `Rails/FilePath` cop error on `join` method with implicit receiver. ([@viralpraxis][])

lib/rubocop/cop/rails/file_path.rb

@@ -66,6 +66,8 @@ def on_dstr(node)
 
         def on_send(node)
           check_for_file_join_with_rails_root(node)
+          return unless node.receiver
+
           check_for_rails_root_join_with_slash_separated_path(node)
           check_for_rails_root_join_with_string_arguments(node)
         end

spec/rubocop/cop/rails/file_path_spec.rb

@@ -236,6 +236,14 @@
         RUBY
       end
     end
+
+    context 'with `join` method with implicit receiver' do
+      it 'does not register an offense' do
+        expect_no_offenses(<<~RUBY)
+          join(Rails.root, path)
+        RUBY
+      end
+    end
   end
 
   context 'when EnforcedStyle is `arguments`' do
@@ -420,5 +428,13 @@
         RUBY
       end
     end
+
+    context 'with `join` method with implicit receiver' do
+      it 'does not register an offense' do
+        expect_no_offenses(<<~RUBY)
+          join(Rails.root, path)
+        RUBY
+      end
+    end
   end
 end