[Transform] fix XPackRestIT continuous transform stats test failure

do not match explicit number but only test existence for duration test (elastic#52504)

fixes elastic#52429

Author: Hendrik Muhs

x-pack/plugin/src/test/resources/rest-api-spec/test/transform/transforms_stats.yml

@@ -247,6 +247,7 @@ teardown:
   - gte: { transforms.0.stats.search_time_in_ms: 0 }
   - lte: { transforms.0.stats.search_total: 1 }
   - match: { transforms.0.stats.search_failures: 0 }
+  - is_true: transforms.0.stats.exponential_avg_checkpoint_duration_ms
   - match: { transforms.0.stats.exponential_avg_documents_indexed: 0.0 }
   - match: { transforms.0.stats.exponential_avg_documents_processed: 0.0 }