[Integrated] RFR: git-pr: set should allow no assignees

[Erik Helin]

Changeset: c309df62
Author:    Erik Helin <ehelin at openjdk.org>
Date:      2020-02-21 10:06:28 +0000
URL:       https://git.openjdk.java.net/skara/commit/c309df62

git-pr: set should allow no assignees

Reviewed-by: rwestberg

! args/src/main/java/org/openjdk/skara/args/Argument.java
! args/src/main/java/org/openjdk/skara/args/ArgumentParser.java
! cli/src/main/java/org/openjdk/skara/cli/pr/GitPrSet.java