Allow management_project_id to be unset

Add test showing the bug.

app/services/clusters/update_service.rb

@@ -23,7 +23,7 @@ module Clusters
     end
 
     def validate_params(cluster)
-      if params[:management_project_id]
+      if params[:management_project_id].present?
         management_project = management_project_scope(cluster).find_by_id(params[:management_project_id])
 
         unless management_project

spec/services/clusters/update_service_spec.rb

@@ -138,6 +138,23 @@ describe Clusters::UpdateService do
             expect(cluster.management_project_id).to be_nil
           end
         end
+
+        context 'cluster already has a management project set' do
+          before do
+            cluster.update!(management_project: create(:project))
+          end
+
+          let(:params) do
+            { management_project_id: '' }
+          end
+
+          it 'unsets management_project_id' do
+            is_expected.to eq(true)
+
+            cluster.reload
+            expect(cluster.management_project_id).to be_nil
+          end
+        end
       end
 
       context 'project cluster' do