[Fix #7171] Fix an error for Style/SafeNavigation

Fixes #7171.

This PR fixes an error for `Style/SafeNavigation` when using `unless nil?`
as a safeguarded.

```ruby
# example.rb
class Object
  # @yield [Object]
  def tap_and_return
    yield self unless nil?
  end
end
```

```console
% rubocop --only Style/SafeNavigation -d
For /private/tmp/7171: configuration from
/Users/koic/.rbenv/versions/2.6.3/lib/ruby/gems/2.6.0/gems/rubocop-0.71.0/config/default.yml
Inspecting 1 file
Scanning /private/tmp/7171/example.rb
An error occurred while Style/SafeNavigation cop was inspecting
/private/tmp/7171/example.rb:4:4.
undefined method `parent' for nil:NilClass
/Users/koic/.rbenv/versions/2.6.3/lib/ruby/gems/2.6.0/gems/rubocop-0.71.0/lib/rubocop/cop/style/safe_navigation.rb:227:in
`method_called?'
/Users/koic/.rbenv/versions/2.6.3/lib/ruby/gems/2.6.0/gems/rubocop-0.71.0/lib/rubocop/cop/style/safe_navigation.rb:111:in
`use_var_only_in_unless_modifier?'
```

CHANGELOG.md

@@ -3,8 +3,8 @@
 ## master (unreleased)
 
 ### New features
-* [#7137](https://github.com/rubocop-hq/rubocop/issues/7137): Add new `Gemspec/RubyVersionGlobalsUsage` cop. ([@malyshkosergey][])
 
+* [#7137](https://github.com/rubocop-hq/rubocop/issues/7137): Add new `Gemspec/RubyVersionGlobalsUsage` cop. ([@malyshkosergey][])
 * [#7150](https://github.com/rubocop-hq/rubocop/pull/7150): Add `AllowIfModifier` option to `Style/IfInsideElse` cop. ([@koic][])
 * [#7153](https://github.com/rubocop-hq/rubocop/pull/7153): Add new cop `Style/FloatDivision` that checks coercion. ([@tejasbubane][])
 
@@ -20,6 +20,7 @@
 * [#7151](https://github.com/rubocop-hq/rubocop/issues/7151): Fix `Style/WordArray` to also consider words containing hyphens. ([@fwitzke][])
 * [#6893](https://github.com/rubocop-hq/rubocop/issues/6893): Handle implicit rescue correctly in `Naming/RescuedExceptionsVariableName`. ([@pocke][], [@anthony-robin][])
 * [#7165](https://github.com/rubocop-hq/rubocop/issues/7165): Fix an auto-correct error for `Style/ConditionalAssignment` when without `else` branch'. ([@koic][])
+* [#7171](https://github.com/rubocop-hq/rubocop/issues/7171): Fix an error for `Style/SafeNavigation` when using `unless nil?` as a safeguarded'. ([@koic][])
 
 ### Changes
 

lib/rubocop/cop/style/safe_navigation.rb

@@ -224,7 +224,7 @@ def negated?(send_node)
         end
 
         def method_called?(send_node)
-          send_node.parent&.send_type?
+          send_node&.parent&.send_type?
         end
 
         def begin_range(node, method_call)

spec/rubocop/cop/style/safe_navigation_spec.rb

@@ -157,6 +157,12 @@
     expect_no_offenses('foo(bar.baz) if bar')
   end
 
+  it 'allows a method call safeguarded when using `unless nil?`' do
+    expect_no_offenses(<<~RUBY)
+      foo unless nil?
+    RUBY
+  end
+
   shared_examples 'all variable types' do |variable|
     context 'modifier if' do
       shared_examples 'safe guarding logical break keywords' do |keyword|