Fix case where "SecretTokenAsPlainText" is null

[jmini]

Fixes #304
Fixes #292

getSecretTokenAsPlainText() can return null, see:
https://github.com/jenkinsci/gitlab-branch-source-plugin/blob/d45c0f4c00428cd0d79ba15af87091708b4b58b8/src/main/java/io/jenkins/plugins/gitlabserverconfig/servers/GitLabServer.java#LL397C1-L407C1

So the code needs to be able to handle the null case.

• When updating the Hook registration in GitLab we need to send "" (since null will just preserve the current value)
• When checking the X-Gitlab-Token sent by GitLab in the POST request, when the header is absent, this means that the SecretToken in Jenkins for that server is expected to be null or empty.

Testing done

TODO

Submitter checklist

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• Ensure that the pull request title represents the desired changelog entry
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• Link to relevant issues in GitHub or Jira
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• Ensure you have provided tests - that demonstrates feature works or fixes the issue

[jetersen]

@jmini Seems there is some compile errors: https://ci.jenkins.io/blue/organizations/jenkins/Plugins%2Fgitlab-branch-source-plugin/detail/PR-324/1/pipeline/44#step-69-log-101

[piotrroda]

@jmini What about your changes? Could you check these errors? This fix is very important. :)

[jmini]

I reverted too much imports changes.

[jmini]

@jetersen I have now tested locally

The <jenkins>/job/<project>/computation/console does not contains a NPE anymore.

I also have tried changing the Secret back and forth in <jenkins>/manage/configure in Manage Jenkins >
Configure System under the GitLab Server server.

The corresponding secret gets updated in GitLab:

And the webhook triggers still works.

Verified as well in the "Scan GitLab Project" view <jenkins>/job/<project>/job/<gitlab-project>/indexing/events