Support beginless and endless ranges in parser

[stackmystack]

Description

Fixes https://github.com/lsegal/yard/issues/1521.

Avoid errors like:

  `first': cannot get the first element of beginless range

Completed Tasks

• [x] I have read the [Contributing Guide][contrib].
• [x] The pull request is complete (implemented / written).
• [x] Git commits have been cleaned up (squash WIP / revert commits).
• [ ] I wrote tests and ran bundle exec rake locally (if code is attached to PR).
  • [x] for ast_node.rb
  • [ ] for ruby_parser.rb : this one is a bit more involved (and may be unnecessary since it touches on the whole test suite), and I don't think I want to commit to writing tests for it if PR altogether is rejected for any reason.

[stackmystack]

Hello, Could you please allow the tests to run again? I fixed them on my branch, and it should work everywhere else.

PS:

In the CI run, runners on macOS+amd64 and ruby < 2.6 do not work. You should remove them I guess?