Fix allow_blank for chat notification validation

Changelog: fixed

app/models/project_services/chat_notification_service.rb

@@ -30,7 +30,7 @@ class ChatNotificationService < Service
   boolean_accessor :notify_only_broken_pipelines, :notify_only_default_branch
 
   validates :webhook, presence: true, public_url: true, if: :activated?
-  validates :labels_to_be_notified_behavior, inclusion: { in: LABEL_NOTIFICATION_BEHAVIOURS }, allow_nil: true
+  validates :labels_to_be_notified_behavior, inclusion: { in: LABEL_NOTIFICATION_BEHAVIOURS }, allow_blank: true
 
   def initialize_properties
     if properties.nil?

changelogs/unreleased/fix-slack-label-filter-behavior-blank.yml

+---
+title: Fix slack label filter behavior blank
+merge_request: 61236
+author:
+type: fixed

spec/models/project_services/chat_notification_service_spec.rb

@@ -12,7 +12,7 @@ RSpec.describe ChatNotificationService do
   end
 
   describe 'validations' do
-    it { is_expected.to validate_inclusion_of(:labels_to_be_notified_behavior).in_array(%w[match_any match_all]) }
+    it { is_expected.to validate_inclusion_of(:labels_to_be_notified_behavior).in_array(%w[match_any match_all]).allow_blank }
   end
 
   describe '#can_test?' do
@@ -127,6 +127,22 @@ RSpec.describe ChatNotificationService do
         end
       end
 
+      context 'when labels_to_be_notified_behavior is blank' do
+        subject(:chat_service) { described_class.new(labels_to_be_notified: label_filter, labels_to_be_notified_behavior: '') }
+
+        context 'no matching labels' do
+          let(:label_filter) { '~some random label' }
+
+          it_behaves_like 'does not notify the chat service'
+        end
+
+        context 'only one label matches' do
+          let(:label_filter) { '~some random label, ~Bug' }
+
+          it_behaves_like 'notifies the chat service'
+        end
+      end
+
       context 'when labels_to_be_notified_behavior is match_any' do
         subject(:chat_service) do
           described_class.new(