Handle nil case typechecking @!parse blocks

[apiology]

I hit this on some real-world code, reproduced it with a spec and fixed it:

  1) Solargraph::TypeChecker strict level complains on @!parse blocks too
     Failure/Error:
       stringified_comments[rng.start.line] ||= begin
         buff = associated_comments[rng.start.line]
         buff ? stringify_comment_array(buff) : nil

     NoMethodError:
       undefined method `start' for nil:NilClass

             stringified_comments[rng.start.line] ||= begin
                                     ^^^^^^
     # ./lib/solargraph/source.rb:256:in `comments_for'
     # ./lib/solargraph/type_checker.rb:546:in `block in without_ignored'
     # ./lib/solargraph/type_checker.rb:543:in `reject'
     # ./lib/solargraph/type_checker.rb:543:in `without_ignored'
     # ./lib/solargraph/type_checker.rb:43:in `problems'
     # ./spec/type_checker/levels/strict_spec.rb:15:in `block (3 levels) in <top (required)>'

[apiology]

@castwide : Let me know if you think this is an issue upstream of the type checker and I can hunt for a better place for the fix - wasn't sure if receiving a nil node in this situation was expected.