Catch more exceptions

[mr-c]

Description

• graph fetching: be more resilient if the graph was created in another thread (java.nio.file.FileAlreadyExistsException in logs )
• if there is a checkout conflict, switch to another directory for the checkout (org.eclipse.jgit.errors.CheckoutConflictException)
• cope with null default values (java.lang.NullPointerException: Cannot invoke "Object.getClass()" because "default_value" is null)
• doc field can also be a list of strings

  java.lang.ClassCastException: class java.util.ArrayList cannot be cast to class java.lang.String (java.util.ArrayList and java.lang.String are in module java.base of loader 'bootstrap')
  at org.commonwl.view.cwl.CWLService.extractDoc(CWLService.java:961)

  Motivation and Context

  Fix many exceptions as seen in the logs

How Has This Been Tested?

• Unit tests added or adjusted for the null default value and the doc as a list of strings

Screenshots (if appropriate):

Types of changes

• [x] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• [ ] My change requires a change to the documentation.
• [ ] I have updated the documentation accordingly.
• [x] I have added tests to cover my changes.
• [ ] All new and existing tests passed.