Fix exported API bucket check. (#8272)

Author: isoos

app/lib/shared/configuration.dart

@@ -423,7 +423,7 @@ final class Configuration {
     publicPackagesBucketName!,
     searchSnapshotBucketName!,
     taskResultBucketName!,
-    exportedApiBucketName!,
+    if (exportedApiBucketName != null) exportedApiBucketName!,
   ]);
 
   late final isProduction = projectId == 'dartlang-pub';

app/test/shared/configuration_test.dart

@@ -77,6 +77,9 @@ void main() {
       if (config.isProduction) {
         expect(rateLimits, hasLength(greaterThan(10)));
       }
+
+      // expect bucket names are non-null
+      expect(config.allBucketNames, isNotEmpty);
     }
   });
 }