Ensure empty strings are coerced to nil when saving ai_gateway_url

ee/app/graphql/mutations/ai/duo_settings/update.rb

@@ -18,7 +18,7 @@ class Update < BaseMutation
         def resolve(**args)
           raise_resource_not_available_error! unless Ability.allowed?(current_user, :manage_self_hosted_models_settings)
 
-          result = ::Ai::DuoSettings::UpdateService.new(ai_gateway_url: args[:ai_gateway_url]).execute
+          result = ::Ai::DuoSettings::UpdateService.new(ai_gateway_url: args[:ai_gateway_url].presence).execute
 
           if result.error?
             {

ee/spec/requests/api/graphql/ai/duo_settings/update_spec.rb

@@ -63,6 +63,21 @@
 
           expect { duo_settings.reload }.to change { duo_settings.ai_gateway_url }.to("http://new-ai-gateway-url")
         end
+
+        context 'when ai_gateway_url arg is a blank string' do
+          let(:mutation_params) { { ai_gateway_url: "" } }
+
+          it 'coerces it to nil' do # an empty string will cause the Duo healthcheck to error
+            request
+
+            result = json_response['data']['duoSettingsUpdate']
+
+            expect(result).to include("aiGatewayUrl" => nil)
+            expect(result['errors']).to eq([])
+
+            expect { duo_settings.reload }.to change { duo_settings.ai_gateway_url }.to(nil)
+          end
+        end
       end
     end
   end