Correctly ignore non-label events in label event handler

[pascal-hofmann]

This fixes the following error that occurs in our jenkins log on each GitHub event that is not of action labeled:

SEVERE  o.j.p.g.e.GHEventsSubscriber$4#applyNullSafe: Subscriber com.adobe.jenkins.github_pr_comment_build.IssueLabelGHEventSubscriber failed to process SCMEvent{type=UPDATED,…} hook, skipping...

net.sf.json.JSONException: null object
at net.sf.json.JSONObject.verifyIsNull(JSONObject.java:2688)
at net.sf.json.JSONObject.getString(JSONObject.java:2035)
at com.adobe.jenkins.github_pr_comment_build.IssueLabelGHEventSubscriber.onEvent(IssueLabelGHEventSubscriber.java:96)
at org.jenkinsci.plugins.github.extension.GHEventsSubscriber.onEvent(GHEventsSubscriber.java:142)
at org.jenkinsci.plugins.github.extension.GHEventsSubscriber$4.applyNullSafe(GHEventsSubscriber.java:241)
at org.jenkinsci.plugins.github.extension.GHEventsSubscriber$4.applyNullSafe(GHEventsSubscriber.java:237)
at org.jenkinsci.plugins.github.util.misc.NullSafeFunction.apply(NullSafeFunction.java:18)

Testing done

Tested on our internal jenkins test instance.

### Submitter checklist
- [x] Make sure you are opening from a **topic/feature/bugfix branch** (right side) and not your main branch!
- [x] Ensure that the pull request title represents the desired changelog entry
- [x] Please describe what you did
- ~~[ ] Link to relevant issues in GitHub or Jira~~
- ~~[ ] Link to relevant pull requests, esp. upstream and downstream changes~~
- ~~[ ] Ensure you have provided tests - that demonstrates feature works or fixes the issue~~

[sparrowt]

Yes! Thank you. This is exactly the kind of change I was about to make having hit the same issue & realised it was exploding on other random events that don't have label in the top-level payload.

[sparrowt]

Apparently I have the ability to approve this - LGTM! 👍 No idea about how a new release of this plugin gets made though.

[bluesliverx]

Thanks for the contribution! I don't remember how new releases get done either (hopefully it's just built into GitHub actions but this is an older repo for me), but I'll work on that.

[bluesliverx]

This is released in 103.vc8919acf2a6b