[Backport 2.11] Don't attempt to parse workflow if it doesn't exist

[opensearch-trigger-bot]

Backport 733fd4e from #1346.

[engechas]

Multinode are failing with known flaky test discussed in #1363

org.opensearch.alerting.resthandler.WorkflowRestApiIT > test chained alerts and audit alerts for workflows with query level monitor FAILED
    java.lang.AssertionError: expected:<0> but was:<2>
        at __randomizedtesting.SeedInfo.seed([841910152E9FE0B:6043A8CD140A6FC]:0)
        at org.junit.Assert.fail(Assert.java:89)
        at org.junit.Assert.failNotEquals(Assert.java:835)
        at org.junit.Assert.assertEquals(Assert.java:120)
        at org.junit.Assert.assertEquals(Assert.java:146)
        at org.opensearch.alerting.resthandler.WorkflowRestApiIT.test chained alerts and audit alerts for workflows with query level monitor(WorkflowRestApiIT.kt:1126)
REPRODUCE WITH: ./gradlew ':alerting:integTest' --tests "org.opensearch.alerting.resthandler.WorkflowRestApiIT.test chained alerts and audit alerts for workflows with query level monitor" -Dtests.seed=841910152E9FE0B -Dtests.security.manager=false -Dtests.locale=id-ID -Dtests.timezone=SystemV/EST5 -Druntime.java=11