[JENKINS-68708] CrowdSecurityRealm: Prevent trim() calls on null strings

[proski]

*  [JENKINS-68708] CrowdSecurityRealm: Prevent trim() calls on null strings
   
   Replace trim() with StringUtils.trimToEmpty(), it trims strings as well
   and also converts null to an empty string.
   
   Don't use trim() in the compatibility constructor. Passwords should be
   used without trimming. Secret.fromString(null) returns a valid object.
   
   Add unit tests for CrowdSecurityRealm constructors.
   
   Signed-off-by: Pavel Roskin <plroskin@gmail.com>

https://issues.jenkins.io/browse/JENKINS-68708

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• Please describe what you did
• 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

Manual testing performed:

• Ran "mvn verify" successfully
• Deployed the resulting crowd2.hpi to a Jenkins 2.346.2 server
• Removed cookieTokenkey, group and httpProxyPassword from jenkins.yaml
• Loaded the modified jenkins.yaml
• Logged out and logged in (crowd2 is used for authentication)
• Downloaded jenkins.yaml
• cookieTokenkey, group and httpProxyPassword are not in the downloaded jenkins.yaml as exprected
• Checked Jenkins log for any potential problems, did not find any.

[DuMaM]

Thanks for this pr.