Fix an error for Lint/UselessRuby2Keywords

This PR fixes the following error for `Lint/UselessRuby2Keywords`
when using `Proc#ruby2_keywords`.

```ruby
block = proc { |_, *args| klass.new(*args) }
block.ruby2_keywords if block.respond_to?(:ruby2_keywords
```

```console
% bundle exec rubocop --only Lint/UselessRuby2Keywords -d
(snip)

An error occurred while Lint/UselessRuby2Keywords cop was inspecting /Users/koic/src/github.com/koic/rubocop-issues/ruby2_keywords/example.rb:2:0.
undefined method `def_type?' for nil:NilClass
/Users/koic/src/github.com/rubocop/rubocop/lib/rubocop/cop/lint/useless_ruby2_keywords.rb:80:in `on_send'
```

changelog/fix_an_error_for_lint_useless_ruby2_keywords.md

@@ -0,0 +1 @@
+* [#11369](https://github.com/rubocop/rubocop/pull/11369): Fix an error for `Lint/UselessRuby2Keywords` when using `Proc#ruby2_keywords`. ([@koic][])

lib/rubocop/cop/lint/useless_ruby2_keywords.rb

@@ -77,10 +77,12 @@ class UselessRuby2Keywords < Base
         PATTERN
 
         def on_send(node)
-          if node.first_argument.def_type?
-            inspect_def(node, node.first_argument)
+          return unless (first_argument = node.first_argument)
+
+          if first_argument.def_type?
+            inspect_def(node, first_argument)
           elsif node.first_argument.sym_type?
-            inspect_sym(node, node.first_argument)
+            inspect_sym(node, first_argument)
           end
         end
 

spec/rubocop/cop/lint/useless_ruby2_keywords_spec.rb

@@ -163,5 +163,12 @@ def foo(**kwargs)
         ^^^^^^^^^^^^^^^^^^^ `ruby2_keywords` is unnecessary for method `foo`.
       RUBY
     end
+
+    it 'does not register an offense for `Proc#ruby2_keywords`' do
+      expect_no_offenses(<<~RUBY)
+        block = proc { |_, *args| klass.new(*args) }
+        block.ruby2_keywords if block.respond_to?(:ruby2_keywords)
+      RUBY
+    end
   end
 end