Spec: applications not yet installed doesn't count

We should not consider an application that is not yet installed or where
the install errored as available. Write a spec to assert this

spec/services/prometheus/adapter_service_spec.rb

@@ -22,6 +22,14 @@ describe Prometheus::AdapterService do
     context "prometheus service can't execute queries" do
       let(:prometheus_service) { double(:prometheus_service, can_query?: false) }
 
+      context 'with cluster with prometheus not yet installed' do
+        let!(:prometheus) { create(:clusters_applications_prometheus, :installable, cluster: cluster) }
+
+        it 'returns nil' do
+          expect(subject.prometheus_adapter).to be_nil
+        end
+      end
+
       context 'with cluster with prometheus installed' do
         let!(:prometheus) { create(:clusters_applications_prometheus, :installed, cluster: cluster) }