feat: Non-existing context entries result in `null`

[saig0]

Description

• Change the behavior to return null if a context doesn't contain an entry with the given key, or if the context is null. Previously, the evaluation failed in this case.
• Write new test cases and adjust the expected behavior of existing test cases. Use the new evaluation result matcher in all test cases to verify reported failures, improve readability, and align the testing style.

Related issues

contributes to #674

[saig0]

@korthout please review the PR in the next 4 weeks (or assign to someone else). :wink:

[saig0]

💭 As this behavior breaks user space (and regresses https://github.com/camunda/feel-scala/issues/695), there must be clear communication to users in the release notes about these changes. Perhaps this requires a major version bump. WDYT?

I agree that we must be very clear with the communication and provide solutions. :+1:

I don't see the need for a major version here. From the spec perspective, the current behavior is a bug and we're fixing the bug. In most cases, the impact on the user should be minor because the is defined() function is used in edge cases.

However, a major version would not help in the context of Camunda 8. I don't think that it would change something about the integration of the FEEL engine.