fix PHP 8 "strlen(null)" when moving paths with missing options

Summary:
Running ./bin/repository move-paths without specifying --from nor --to
leads to the PHP 8 error about strlen() not accepting null parameters.

Test Plan: Running ./bin/repository move-paths without --from/--to (shows a proper error)

Reviewers: O1 Blessed Committers, speck, valerio.bozzolan

Reviewed By: O1 Blessed Committers, speck, valerio.bozzolan

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

Differential Revision: https://we.phorge.it/D25484