Don't validate Jenkins username if password is blank

Users attempting to activate Jenkins with an empty username
and password would still encounter a validation error because
it appeared that the password was changed from `nil` to an
empty string. Relax this validation to check the
password must actually have contents to require a username.

Closes https://gitlab.com/gitlab-org/gitlab-ee/issues/9246

ee/app/models/project_services/jenkins_service.rb

@@ -7,7 +7,7 @@ class JenkinsService < CiService
 
   validates :jenkins_url, presence: true, url: true, if: :activated?
   validates :project_name, presence: true, if: :activated?
-  validates :username, presence: true, if: ->(service) { service.activated? && service.password_touched? }
+  validates :username, presence: true, if: ->(service) { service.activated? && service.password_touched? && service.password.present? }
 
   default_value_for :push_events, true
   default_value_for :merge_requests_events, false

ee/changelogs/unreleased/sh-fix-issue-9246.yml

+---
+title: Don't validate Jenkins username if password is blank
+merge_request: 9198
+author:
+type: fixed

ee/spec/models/project_services/jenkins_service_spec.rb

@@ -60,6 +60,15 @@ describe JenkinsService do
 
         it { is_expected.to validate_presence_of :username }
       end
+
+      context 'when password is blank' do
+        it 'does not validate the username' do
+          expect(subject).not_to validate_presence_of :username
+
+          subject.password = ''
+          subject.save
+        end
+      end
     end
 
     context 'when the service is inactive' do