test(e2e-metrics): add null check for metrics response

- Add null check before accessing response.Metrics.Count to prevent potential NullReferenceException
- Add explicit Assert.NotNull(response.Metrics) assertion before count validation
- Improve test robustness by ensuring metrics collection is not null before operations
- Prevents test failures due to unexpected null responses from CloudWatch API

Author: hjgraca

libraries/tests/e2e/functions/core/metrics/Function/test/Function.Tests/FunctionTests.cs

@@ -142,7 +142,7 @@ private async Task AssertCloudWatch()
             try
             {
                 response = await cloudWatchClient.ListMetricsAsync(request);
-                if (response.Metrics.Count > 6)
+                if (response.Metrics != null && response.Metrics.Count > 6)
                 {
                     break;
                 }
@@ -155,6 +155,7 @@ private async Task AssertCloudWatch()
             await Task.Delay(5000); // wait for 5 seconds before retrying
         }
 
+        Assert.NotNull(response.Metrics);
         Assert.Equal(7, response.Metrics.Count);
 
         foreach (var metric in response.Metrics)