Fix an error for Style/IfUnlessModifier when the body is a method call with hash splat

[fatkodima]

For a code like

if condition
  do_this(**options)
end

I got

     Failure/Error:
       if if_body.call_type? &&
          if_body.last_argument&.hash_type? && if_body.last_argument.pairs.last.value_omission?

     NoMethodError:
       undefined method `value_omission?' for nil:NilClass
     # ./lib/rubocop/cop/mixin/statement_modifier.rb:59:in `if_body_source'
     # ./lib/rubocop/cop/mixin/statement_modifier.rb:51:in `to_modifier_form'
     # ./lib/rubocop/cop/mixin/statement_modifier.rb:46:in `length_in_modifier_form'
     # ./lib/rubocop/cop/mixin/statement_modifier.rb:40:in `modifier_fits_on_single_line?'
     # ./lib/rubocop/cop/mixin/statement_modifier.rb:16:in `single_line_as_modifier?'

[koic]

Thanks!