Fix Performance/CaseWhenSplat cop error on when node without body

[viralpraxis]

Previously if when node has an empty body this cop raises:

case 1
when 1 # no body
  # 1 == 1
else
  nil
end

undefined method `loc' for nil
    # ./lib/rubocop/cop/performance/case_when_splat.rb:154:in `indent_for'
    # ./lib/rubocop/cop/performance/case_when_splat.rb:144:in `new_branch_without_then'
    # ./lib/rubocop/cop/performance/case_when_splat.rb:123:in `reordering_correction'
    # ./lib/rubocop/cop/performance/case_when_splat.rb:114:in `reorder_condition'
    # ./lib/rubocop/cop/performance/case_when_splat.rb:86:in `autocorrect'

It also did not preserve comments.

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