[lib] Get rid of emptyArray in thread-reducer

Summary:
This isn't used anywhere else and doesn't seem necessary? Seems like an unnecesary indirection instead of just placing an empty array (eg [])

Depends on D4195

Test Plan: Didn't test

Reviewers: def-au1t, palys-swm

Reviewed By: palys-swm

Subscribers: ashoat, Adrian, yayabosh

Differential Revision: https://phab.comm.dev/D4196