Fix an error for Style/RequireOrder

[koic]

This PR fixes the following error for Style/RequireOrder when only one reuqire.

require 'a'

% bundle exec rubocop example.rb --only Style/RequireOrder -d
(snip)

undefined method `if_type?' for nil:NilClass
/Users/koic/src/github.com/rubocop/rubocop/lib/rubocop/cop/style/require_order.rb:102:in `not_modifier_form?'
/Users/koic/src/github.com/rubocop/rubocop/lib/rubocop/cop/style/require_order.rb:85:in `on_send'

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• 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!