Always return query parameters

[samuel-williams-shopify]

Summary

• make Reference#parse_query! create and return an empty mutable parameter hash when no query or parameters exist
• document the behavior
• verify mutations are retained by the reference and reflected by serialization

Testing

• bundle exec bake test (391 tests, 573 assertions)
• bundle exec rubocop lib/protocol/url/reference.rb test/protocol/url/reference.rb
• bundle exec bake decode:index:coverage lib (101/101 documented)
• git diff --check