RFR: git-pr: set should allow no assignees

[Erik Helin]

Hi all,

please review this small patch that allows the `--assignees` option to `git pr
set` to be empty. That is, `git pr set --assignees=` should make the pull
request have no user assigned to it.

Testing:
- Manual testing of `git pr set --assignees=` on Linux x64

Thanks,
Erik

-------------

Commits:
 - 283c6fdd: git-pr: set should allow no assignees

Changes: https://git.openjdk.java.net/skara/pull/462/files
 Webrev: https://webrevs.openjdk.java.net/skara/462/webrev.00
  Stats: 5 lines in 3 files changed: 2 ins; 0 del; 3 mod
  Patch: https://git.openjdk.java.net/skara/pull/462.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/462/head:pull/462

PR: https://git.openjdk.java.net/skara/pull/462