AdminEmailsWorker doesn't work when no group is specified

We let you send an email to all users on the instance, or a whole group: https://docs.gitlab.com/ee/tools/email.html

This form doesn't validate that the recipient group field is set:

image

That field allows you to select all users:

image

But if you don't select anything, the background job will fail:

AdminEmailsWorker JID-ae12ec6dd0371d96a7555c4e INFO: arguments: ["","Test","Test"]
AdminEmailsWorker JID-ae12ec6dd0371d96a7555c4e INFO: fail: 0.283 sec
WARN: {"context":"Job raised exception","job":{"class":"AdminEmailsWorker","args":["","Test","Test"],"retry":3,"queue":"admin_emails","backtrace":true,"jid":"ae12ec6dd0371d96a7555c4e","created_at":1569327854.9036999,"correlation_id":"7286E8br3z5","uber-trace-id":"7120c35289c04d8e:4b936e3ee5eb42f2:ef2eab911e9999a7:1","enqueued_at":1569327854.914627},"jobstr":"{\"class\":\"AdminEmailsWorker\",\"args\":[\"\",\"Test\",\"Test\"],\"retry\":3,\"queue\":\"admin_emails\",\"backtrace\":true,\"jid\":\"ae12ec6dd0371d96a7555c4e\",\"created_at\":1569327854.9036999,\"correlation_id\":\"7286E8br3z5\",\"uber-trace-id\":\"7120c35289c04d8e:4b936e3ee5eb42f2:ef2eab911e9999a7:1\",\"enqueued_at\":1569327854.914627}"}
WARN: NoMethodError: undefined method `pluck' for nil:NilClass
WARN: /Users/seanmcgivern/Code/gdk/gitlab/ee/app/workers/admin_emails_worker.rb:8:in `perform'

I think https://gitlab.com/gitlab-org/gitlab/blob/v12.3.0-ee/ee%2Fapp%2Fworkers%2Fadmin_emails_worker.rb#L16-25 should make 'all' the default case, or we should add a form validation.