jq `unique` produces null from empty arrays

Describe the bug

During a gh pr list with jq filtering, any empty array passed to the unique operator outputs null.

$ gh --version
gh version 2.4.0+dfsg1 (2022-03-23 Ubuntu 2.4.0+dfsg1-2)
https://github.com/cli/cli/releases/latest

Steps to reproduce the behavior

1. gh pr list --json title -q '[] | unique'
2. Observe that the output is null

Expected vs actual behavior

Expected output is the empty array ([]). Instead, null is produced.

Logs

$ gh pr list --json title -q '[] | unique'
null