[bugfix][converter/core]: Empty error code leads to conversion error

[jonathanlukas]

Camunda Community Hub Pull Request Template

Description

In Camunda 7, it is ok to leave out the error code in a error to create a catch-all error.

This could not be converted as it lead to a null-pointer exception.

Additional context

closes #315

Testing your changes

There is a new test added asserting the correct behaviour.

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   2m 31s ⏱️ + 2m 31s
190 tests +190  190 ✔️ +190  0 💤 ±0  0 ❌ ±0 
308 runs  +308  308 ✔️ +308  0 💤 ±0  0 ❌ ±0 

Results for commit a708276. ± Comparison against base commit f79df84.

[ingorichtsmeier]

Looks good to me!