Fix PHP 8.1 trim(null) exception in ./bin/diviner generate

Summary:
When running './bin/diviner generate' under PHP 8.1, we get a trim(null) exception.

Fixes T15548

Test Plan: Run './bin/diviner generate'

Reviewers: O1 Blessed Committers, avivey

Reviewed By: O1 Blessed Committers, avivey

Subscribers: avivey, speck, tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Maniphest Tasks: T15548

Differential Revision: https://we.phorge.it/D25353