Fix a ModularTransactions exception with custom fields that support change details

Summary: We're throwing here when we actually want to return null so we make it into custom field handling code. See Conpherence.

Test Plan: Found a failing task and re-executed it with bin/worker execute --id <id>; after this change, it didn't fatal.

Reviewers: chad

Reviewed By: chad

Differential Revision: https://secure.phabricator.com/D17051