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 73.12%. Comparing base (7b42906) to head (f2dfc3e).
Report is 38 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     72.04%   73.12%   +1.08%     
- Complexity      235      245      +10     
============================================
  Files            18       18              
  Lines          1073     1079       +6     
  Branches        149      148       -1     
============================================
+ Hits            773      789      +16     
+ Misses          207      197      -10     
  Partials         93       93              

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[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

[eva-mueller-coremedia]

Sorry for nudging again - @jtnord @michael-doubez would be great if you could have a look.

[michael-doubez]

LGTM but the change is very big and I'd prefer if @jtnord had a look

[eva-mueller-coremedia]

LGTM but the change is very big and I'd prefer if @jtnord had a look

I rebased onto the default branch. Commit-wise review is suggested. This PR contains a fix and just some splitting up of the Plugin Test class into smaller pieces resp. outsources helper / mock methods to own classes.