bugfix: crash when Cucumber::Messages::Group#children is nil (#1819)

This change fixes the `undefined method 'map' for nil (NoMethodError)`
for `cucumber-10.2.0/lib/cucumber/formatter/message_builder.rb:144`:
`'Cucumber::Formatter::MessageBuilder#argument_group_to_message'`
caused by:

* Dec 11, 2025: #1805
  allows to pull `cucumber-cucumber-expressions` `19.0.0`

* Jan 21, 2026: cucumber/messages#370
  introduce a breaking change that allows the `children` field of
  `Cucumber::Messages::Group` to be `nil`

* Jan 21, 2026: cucumber/cucumber-expressions#387
  `ruby/lib/cucumber/cucumber_expressions/group_builder.rb` sets
  the `children` to `nil` if `empty?`

* Jan 26, 2026: The `cucumber-cucumber-expressions` `19.0.0` gem released
  and will be pulled in by `cucumber` `10.2.0` that raises the error

The fix is simply accept the `nil` value for
`Cucumber::Messages::Group#children`.

Author: vizv

CHANGELOG.md

@@ -15,6 +15,9 @@ Please visit [cucumber/CONTRIBUTING.md](https://github.com/cucumber/cucumber/blo
 > This is currently not consumed anywhere, but will become the building blocks for the future of cucumber formatters
 > which we hope to begin migrating to in the start of 2026
 
+### Fixed
+- Fix crash when `Cucumber::Messages::Group#children` is `nil`
+
 ## [10.2.0] - 2025-12-10
 ### Changed
 - Permit the latest version of the `cucumber-html-formatter` (v22.0.0+)

lib/cucumber/formatter/message_builder.rb

@@ -143,7 +143,7 @@ def argument_group_to_message(group)
         Cucumber::Messages::Group.new(
           start: group.start,
           value: group.value,
-          children: group.children.map { |child| argument_group_to_message(child) }
+          children: group.children&.map { |child| argument_group_to_message(child) }
         )
       end