Fix an error for Lint/UselessRuby2Keywords

[koic]

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

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

% 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'

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Before submitting the PR make sure the following are checked:

• The PR relates to only one subject with a clear title and description in grammatically correct, complete sentences.
• Wrote good commit messages.
• Commit message starts with [Fix #issue-number] (if the related issue exists).
• Feature branch is up-to-date with master (if not - rebase it).
• Squashed related commits together.
• Added tests.
• Ran bundle exec rake default. It executes all tests and runs RuboCop on its own code.
• Added an entry (file) to the changelog folder named {change_type}_{change_description}.md if the new code introduces user-observable changes. See changelog entry format for details.

[bbatsov]

Thanks!