Fix an error for Style/RequireOrder when using require inside rescue body

[fatkodima]

For the following code

begin
rescue
  require "foo"
end

I got

undefined method `if_type?' for nil:NilClass

          node.if_type? && !node.modifier_form?
              ^^^^^^^^^
rubocop/lib/rubocop/cop/style/require_order.rb:102:in `not_modifier_form?'
rubocop/lib/rubocop/cop/style/require_order.rb:121:in `sibling_node'
rubocop/lib/rubocop/cop/style/require_order.rb:107:in `block in find_previous_older_sibling'
rubocop/lib/rubocop/cop/style/require_order.rb:106:in `each'
rubocop/lib/rubocop/cop/style/require_order.rb:106:in `find'
rubocop/lib/rubocop/cop/style/require_order.rb:106:in `find_previous_older_sibling'
rubocop/lib/rubocop/cop/style/require_order.rb:87:in `on_send'
rubocop/lib/rubocop/cop/commissioner.rb:137:in `public_send'

Node#left_siblings returns all (sibling) children of the parent node (not only of AST::Node type), but we should work only with nodes.

[bbatsov]

Thanks!