Fix an error for Style/IdenticalConditionalBranches

[koic]

This PR fixes the following error for Style/IdenticalConditionalBranches when using empty parentheses in the if branch:

$ cat example.rb
if condition
  ()
else
  foo
end

$ bundle exec rubocop --only Style/IdenticalConditionalBranches -d
(snip)

An error occurred while Style/IdenticalConditionalBranches cop was inspecting
/Users/koic/src/github.com/koic/rubocop-issues/identical_conditional_branches/example.rb:1:0.
undefined method `assignment?' for nil:NilClass
/Users/koic/src/github.com/rubocop/rubocop/lib/rubocop/cop/style/identical_conditional_branches.rb:161:in `duplicated_expressions?'
/Users/koic/src/github.com/rubocop/rubocop/lib/rubocop/cop/style/identical_conditional_branches.rb:146:in `check_branches'
/Users/koic/src/github.com/rubocop/rubocop/lib/rubocop/cop/style/identical_conditional_branches.rb:120:in `on_if'

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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.