fix: handle null escalation code

[jonathanlukas]

Camunda Community Hub Pull Request Template

Description

This PR adds a null check before entering the expression transformation result

Additional context

Issue appeared in hosted version

Testing your changes

A test has been added to prove that the converter can handle a null escalation code

Types of changes

• Bug fix (non-breaking change which fixes an existing open issue)
• New feature (non-breaking change which adds functionality to an extension)
• Breaking change (fix or feature that would cause existing functionality of an extension to change)
• Documentation update (changes made to an existing piece of documentation)

Checklist:

• My code adheres to the syntax used by this extension.
• My pull request requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the Camunda Community Hub documentation.
• I have read the Pull Request Process documentation.
• I have added or suggested tests to cover my changes suggested in this pull request.
• All new and existing CI/CD tests passed.
• I will /assign myself this issue, and add the relevant [issue labels] to it if they are not automatically generated by Probot.
• I will tag @camunda-community-hub/devrel in a new comment on this issue if 30 days have passed since my pull request was opened and I have not received a response from the extension's maintainer.

[github-actions]

Test Results

 25 files  + 25   25 suites  +25   1m 23s ⏱️ + 1m 23s
234 tests +234  234 ✅ +234  0 💤 ±0  0 ❌ ±0 
350 runs  +350  350 ✅ +350  0 💤 ±0  0 ❌ ±0 

Results for commit fd28c7b. ± Comparison against base commit b67630d.