[incubator-kie-issues-989] Add a null check on milestones attribute in ProcessDetails component

[treivize]

Closes: apache/incubator-kie-issues#989

Building and running the data-index service and the management console based on master branch, it seems that milestones attribute can be now null instead of being an empty array as before.

I have the feeling that it is following this big refactor on the serialization of the process instance feed between the kogito runtime and the data-index service: ffc8500

A simple optional chaining operator is resolving the issue.

[yesamer]

@treivize Thank you for providing this patch and for following our fixing process (opening a ticket and its related PR), we appreciate it a lot!
However, the code that impacts your change will be soon moved into another repository (apache/incubator-kie-tools#2180). Moreover, that PR already contains a null guard
After the merge of that PR, the involved code will be removed from this kogito-apps repo.
With these premises, I guess doesn't make sense to merge your PR, despite your change being correct.
In any case, I invite you to double-check if the bug you spotted still occurs after the PR merge, if that is the case please feel free to open a new PR in the kie-tools repo.

Thank you again!

[treivize]

@yesamer, waiting for the PR to be merged eagerly to test from there. Any idea when it will be completed?
Feel free to close this PR anytime.

[yesamer]

@treivize The ETA is ASAP :), it's an high-priority task. However, that task has a huge impact on our code base, so we don't have a precise ETA for that.

[yesamer]

@treivize The above PR has been merged. Please check and close the kie-issues ticket if the fix is fine. Closing this. Thank you again for your effort.