Ensure Process `user_id` is nullified when referenced EPerson is deleted

[tdonohue]

References

• Fixes backend issues found in https://github.com/DSpace/dspace-angular/pull/3014#pullrequestreview-2044224424

Description

When an EPerson who created a Process is deleted, a reference to the deleted EPerson remains in the process user_id column. This fixes that by nullifying any references to deleted EPerson objects and adding a constraint to ensure they are nullified automatically in the future.

Also fixes a bug in ProcessConverter to ensure when EPerson is null, a NullPointerException doesn't occur.

Instructions for Reviewers

• To be tested with https://github.com/DSpace/dspace-angular/pull/3014 This PR should allow the Process Details page to also work even if the EPerson (who created the process) is deleted.

[tdonohue]

Tested this thoroughly with https://github.com/DSpace/dspace-angular/pull/3014#pullrequestreview-2046442914 as this PR is required to fix all bugs previously reported in https://github.com/DSpace/dspace-angular/pull/3014#pullrequestreview-2044224424

Merging immediately as it's required for https://github.com/DSpace/dspace-angular/pull/3014

[dspace-bot]

Successfully created backport PR for dspace-7_x:

• 9554