[server] Don't throw no_parent_thread_specified if parentThreadID not set
[server] Don't throw no_parent_thread_specified if parentThreadID not set

Summary: Accidentally messed this up in D344. Should've tested better. This was breaking whenever I called `editThread` without setting `changes.parentThreadID`.

Test Plan: Call `editThread` and make sure it doesn't crash

Reviewers: palys-swm

Subscribers: KatPo, zrebcu411, Adrian

Differential Revision: https://phabricator.ashoat.com/D353

Ashoat/squadcal,Ashoat/squadcal,Ashoat/squadcal,Ashoat/squadcal,Ashoat/squadcal,Ashoat/squadcal,Ashoat/squadcal