[Fix rubocop#6882] Fix an error for Rails/RedundantAllowNil

Fixes rubocop#6882.

This PR fixes the following error for `Rails/RedundantAllowNil`
when not using both `allow_nil` and `allow_blank`.

```ruby
% cat app/models/user.rb
class User < ApplicationRecord
  validates :email, presence: true
end
```

```console
% rubocop app/models/user.rb --only Rails/RedundantAllowNil -d
For /private/tmp/6882: configuration from
/Users/koic/.rbenv/versions/2.6.1/lib/ruby/gems/2.6.0/gems/rubocop-0.67.1/config/default.yml
Inspecting 1 file
Scanning /private/tmp/6882/app/models/user.rb
An error occurred while Rails/RedundantAllowNil cop was inspecting
/private/tmp/6882/app/models/user.rb:2:2.
undefined method `children' for nil:NilClass
/Users/koic/.rbenv/versions/2.6.1/lib/ruby/gems/2.6.0/gems/rubocop-0.67.1/lib/rubocop/cop/rails/redundant_allow_nil.rb:42:in `on_send'

(snip)

1 file inspected, no offenses detected

1 error occurred:
An error occurred while Rails/RedundantAllowNil cop was inspecting
/private/tmp/6882/app/models/user.rb:2:2.
Errors are usually caused by RuboCop bugs.
Please, report your problems to RuboCop's issue tracker.
https://github.com/rubocop-hq/rubocop/issues

Mention the following information in the issue report:
0.67.1 (using Parser 2.6.2.0, running on ruby 2.6.1 x86_64-darwin17)
```

CHANGELOG.md

@@ -2,6 +2,10 @@
 
 ## master (unreleased)
 
+### Bug fixes
+
+* [#6882](https://github.com/rubocop-hq/rubocop/issues/6882): Fix an error for `Rails/RedundantAllowNil` when not using both `allow_nil` and `allow_blank`. ([@koic][])
+
 ## 0.67.1 (2019-04-04)
 
 ### Changes

lib/rubocop/cop/rails/redundant_allow_nil.rb

@@ -39,14 +39,12 @@ def on_send(node)
           return unless node.method_name == :validates
 
           allow_nil, allow_blank = find_allow_nil_and_allow_blank(node)
+          return unless allow_nil && allow_blank
+
           allow_nil_val = allow_nil.children.last
           allow_blank_val = allow_blank.children.last
 
-          if allow_nil_val.type == allow_blank_val.type
-            add_offense(allow_nil, message: MSG_SAME)
-          elsif allow_nil_val.false_type? && allow_blank_val.true_type?
-            add_offense(allow_nil, message: MSG_ALLOW_NIL_FALSE)
-          end
+          offense(allow_nil_val, allow_blank_val, allow_nil)
         end
 
         def autocorrect(node)
@@ -66,6 +64,14 @@ def autocorrect(node)
 
         private
 
+        def offense(allow_nil_val, allow_blank_val, allow_nil)
+          if allow_nil_val.type == allow_blank_val.type
+            add_offense(allow_nil, message: MSG_SAME)
+          elsif allow_nil_val.false_type? && allow_blank_val.true_type?
+            add_offense(allow_nil, message: MSG_ALLOW_NIL_FALSE)
+          end
+        end
+
         def find_allow_nil_and_allow_blank(node)
           allow_nil = nil
           allow_blank = nil

spec/rubocop/cop/rails/redundant_allow_nil_spec.rb

@@ -3,6 +3,30 @@
 RSpec.describe RuboCop::Cop::Rails::RedundantAllowNil do
   subject(:cop) { described_class.new }
 
+  context 'when not using both `allow_nil` and `allow_blank`' do
+    it 'registers no offense' do
+      expect_no_offenses(<<-RUBY.strip_indent)
+        validates :email, presence: true
+      RUBY
+    end
+  end
+
+  context 'when using only `allow_nil`' do
+    it 'registers no offense' do
+      expect_no_offenses(<<-RUBY.strip_indent)
+        validates :email, allow_nil: true
+      RUBY
+    end
+  end
+
+  context 'when using only `allow_blank`' do
+    it 'registers no offense' do
+      expect_no_offenses(<<-RUBY.strip_indent)
+        validates :email, allow_blank: true
+      RUBY
+    end
+  end
+
   context 'when both allow_nil and allow_blank are true' do
     it 'registers an offense' do
       expect_offense(<<-RUBY.strip_indent)