dev/core#470: Current employer disapears when disabling expired relationships

dev/core#470: Current employer disapears when disabling expired relationships

Overview
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Steps to replicate:

1. Create a current relationship (set empty end-date) on individual A as  organization A and mark as current employer
2. Create a past relationship on individual A as  organization B without marking as current employer
3. Run Disable expired relationships cron job

Before
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Result: Clear the employer field of Contact A

After
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Result: Does not clear the employer field of Contact A

Technical Details
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This PR replaces https://github.com/civicrm/civicrm-core/pull/13334