[release-19.0] VReplication: Properly handle target shards w/o a primary in Reshard (#16283)#16291

mattlord merged 2 commits intorelease-19.0vitessio/vitess:release-19.0from

backport-16283-to-release-19.0vitessio/vitess:backport-16283-to-release-19.0Copy head branch name to clipboard