Prevent NullPointerException when using API token w/o OIC session

[eva-mueller-coremedia]

I encountered warnings in my IDE about potential NPEs and fixed them accordingly.

In the course of this small fix, I tried to understand the integration test in PluginTest.java and found it a little bit ... grown.
Thus I made some housekeeping

• move mock methods to own file
• move helper methods to own file
• extracted topic related test from PluginTest.java into own test files

Testing done

This change has solely tested by the integration test in PluginApiTokenTest.java.

Submitter checklist

• Make sure you are opening from a topic/feature/bugfix branch (right side) and not your main branch!
• Ensure that the pull request title represents the desired changelog entry
• 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

[codecov]

Codecov Report

Attention: Patch coverage is 78.94737% with 4 lines in your changes missing coverage. Please review.

Project coverage is 73.05%. Comparing base (84359ac) to head (da564b7).
Report is 8 commits behind head on master.

Files with missing lines	Patch %	Lines
...va/org/jenkinsci/plugins/oic/OicSecurityRealm.java	78.94%	0 Missing and 4 partials ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master     #483      +/-   ##
============================================
+ Coverage     71.73%   73.05%   +1.31%     
- Complexity      222      233      +11     
============================================
  Files            17       17              
  Lines          1033     1039       +6     
  Branches        148      147       -1     
============================================
+ Hits            741      759      +18     
+ Misses          201      190      -11     
+ Partials         91       90       -1     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[eva-mueller-coremedia]

@jtnord @michael-doubez would be great if you could have a look.
Unfortunately, I am not allowed to use @jenkinsci/oic-auth-plugin-developers