test(retry): add validation to ensure at least one test case is resol…

…ved (#1109)

* If an overly specific allowed filter can prevent any test cases from being passed to the runner, add validation to ensure there is at least one test case.

google-cloud-storage/src/test/java/com/google/cloud/storage/conformance/retry/ITRetryConformanceTest.java

@@ -20,6 +20,7 @@
 import static com.google.cloud.storage.PackagePrivateMethodWorkarounds.bucketCopyWithStorage;
 import static com.google.cloud.storage.conformance.retry.Ctx.ctx;
 import static com.google.cloud.storage.conformance.retry.State.empty;
+import static com.google.common.truth.Truth.assertThat;
 import static java.util.Objects.requireNonNull;
 import static org.junit.Assert.assertNotNull;
 
@@ -151,7 +152,9 @@ public static Collection<Object[]> testCases() throws IOException {
             .setTestAllowFilter(RetryTestCaseResolver.includeAll())
             .build();
 
-    return resolver.getRetryTestCases().stream()
+    List<RetryTestCase> retryTestCases = resolver.getRetryTestCases();
+    assertThat(retryTestCases).isNotEmpty();
+    return retryTestCases.stream()
         .map(rtc -> new Object[] {rtc.testRetryConformance, rtc.rpcMethodMapping})
         .collect(ImmutableList.toImmutableList());
   }