Fix 500 error when visiting a non-existing integration

Changelog: fixed

app/models/project.rb

@@ -1484,7 +1484,7 @@ class Project < ApplicationRecord
   end
 
   def find_or_initialize_integration(name)
-    return if disabled_integrations.include?(name)
+    return if disabled_integrations.include?(name) || Integration.available_integration_names.exclude?(name)
 
     find_integration(integrations, name) || build_from_instance(name) || build_integration(name)
   end

spec/models/project_spec.rb

@@ -6288,6 +6288,10 @@ RSpec.describe Project, factory_default: :keep do
       expect(subject.find_or_initialize_integration('prometheus')).to be_nil
     end
 
+    it 'returns nil if integration does not exist' do
+      expect(subject.find_or_initialize_integration('non-existing')).to be_nil
+    end
+
     context 'with an existing integration' do
       subject { create(:project) }