Add nil check to manifest_route_update_spec for better error messages

The test was still occasionally failing with 'undefined method protocol
for nil:NilClass' even after the dataset fix. This adds an explicit
nil check to provide a clearer failure message if the route mapping
is unexpectedly nil, making it easier to debug the race condition.

Author: joyvuu-dave

spec/unit/actions/manifest_route_update_spec.rb

@@ -69,7 +69,9 @@ module VCAP::CloudController
               route_mappings = app.reload.route_mappings_dataset.where(route:)
 
               expect(route_mappings.count).to eq(1)
-              expect(route_mappings.first.protocol).to eq('http2')
+              mapped_route = route_mappings.first
+              expect(mapped_route).not_to be_nil
+              expect(mapped_route.protocol).to eq('http2')
             end
           end
         end