Fix TestBulkMigrator.testBulkMigrationStartStopProcessor randomness

[2023-05-04T08:07:56.569Z] [ERROR] Tests run: 3, Failures: 0, Errors: 1, Skipped: 0, Time elapsed: 6.203 s <<< FAILURE! - in org.nuxeo.ecm.core.migrator.TestBulkMigrator
[2023-05-04T08:07:56.569Z] [ERROR] testBulkMigrationStartStopProcessor(org.nuxeo.ecm.core.migrator.TestBulkMigrator)  Time elapsed: 1.203 s  <<< ERROR!
[2023-05-04T08:07:56.569Z] java.lang.NumberFormatException: Cannot parse null string
[2023-05-04T08:07:56.569Z] 	at java.base/java.lang.Long.parseLong(Long.java:674)
[2023-05-04T08:07:56.569Z] 	at java.base/java.lang.Long.parseLong(Long.java:836)
[2023-05-04T08:07:56.569Z] 	at org.nuxeo.runtime.migration.MigrationServiceImpl.getStatus(MigrationServiceImpl.java:387)
[2023-05-04T08:07:56.569Z] 	at org.nuxeo.ecm.core.migrator.TestBulkMigrator.lambda$testBulkMigrationStartStopProcessor$4(TestBulkMigrator.java:146)
[2023-05-04T08:07:56.569Z] 	at org.awaitility.core.CallableCondition$ConditionEvaluationWrapper.eval(CallableCondition.java:100)
[2023-05-04T08:07:56.569Z] 	at org.awaitility.core.ConditionAwaiter$ConditionPoller.call(ConditionAwaiter.java:201)
[2023-05-04T08:07:56.569Z] 	at org.awaitility.core.ConditionAwaiter$ConditionPoller.call(ConditionAwaiter.java:188)
[2023-05-04T08:07:56.569Z] 	at java.base/java.util.concurrent.FutureTask.run(FutureTask.java:264)
[2023-05-04T08:07:56.569Z] 	at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1136)
[2023-05-04T08:07:56.569Z] 	at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:635)
[2023-05-04T08:07:56.569Z] 	at java.base/java.lang.Thread.run(Thread.java:833)