Don't attempt to parse workflow if it doesn't exist

[engechas]

Issue #, if available:
Partially opensearch-project/security-analytics#618

Description of changes:
The DeleteWorkflow action attempts to parse the Workflow object when reading it before delete, even if the Workflow was not found. This leads to an NPE:

[2023-12-21T11:55:00,674][ERROR][o.o.a.t.TransportDeleteWorkflowAction] [integTest-0] Failed to delete workflow WWvxjYwBC1VdWUdNmDay
java.lang.NullPointerException: Cannot invoke "org.opensearch.core.common.bytes.BytesReference.length()" because "bytes" is null
        at org.opensearch.compress.ZstdCompressor.isCompressed(ZstdCompressor.java:66) ~[opensearch-compress-3.0.0-SNAPSHOT.jar:3.0.0-SNAPSHOT]
        at org.opensearch.core.compress.CompressorRegistry.compressor(CompressorRegistry.java:65) ~[opensearch-core-3.0.0-SNAPSHOT.jar:3.0.0-SNAPSHOT]
        at org.opensearch.common.xcontent.XContentHelper.createParser(XContentHelper.java:109) ~[opensearch-3.0.0-SNAPSHOT.jar:3.0.0-SNAPSHOT]
        at org.opensearch.alerting.transport.TransportDeleteWorkflowAction$DeleteWorkflowHandler.getWorkflow(TransportDeleteWorkflowAction.kt:310) ~[opensearch-alerting-3.0.0.0-SNAPSHOT.jar:3.0.0.0-SNAPSHOT]
        at org.opensearch.alerting.transport.TransportDeleteWorkflowAction$DeleteWorkflowHandler.access$getWorkflow(TransportDeleteWorkflowAction.kt:112) ~[opensearch-alerting-3.0.0.0-SNAPSHOT.jar:3.0.0.0-SNAPSHOT]
        at org.opensearch.alerting.transport.TransportDeleteWorkflowAction$DeleteWorkflowHandler$getWorkflow$1.invokeSuspend(TransportDeleteWorkflowAction.kt) ~[opensearch-alerting-3.0.0.0-SNAPSHOT.jar:3.0.0.0-SNAPSHOT]
        at kotlin.coroutines.jvm.internal.BaseContinuationImpl.resumeWith(ContinuationImpl.kt:33) [kotlin-stdlib-1.8.21.jar:1.8.21-release-380(1.8.21)]
        at kotlinx.coroutines.DispatchedTask.run(Dispatched.kt:233) [kotlinx-coroutines-core-1.1.1.jar:?]
        at kotlinx.coroutines.scheduling.CoroutineScheduler.runSafely(CoroutineScheduler.kt:594) [kotlinx-coroutines-core-1.1.1.jar:?]
        at kotlinx.coroutines.scheduling.CoroutineScheduler.access$runSafely(CoroutineScheduler.kt:60) [kotlinx-coroutines-core-1.1.1.jar:?]
        at kotlinx.coroutines.scheduling.CoroutineScheduler$Worker.run(CoroutineScheduler.kt:742) [kotlinx-coroutines-core-1.1.1.jar:?]
[2023-12-21T11:55:00,684][ERROR][o.o.a.u.AlertingException] [integTest-0] Alerting error: java.lang.NullPointerException: Cannot invoke "org.opensearch.core.common.bytes.BytesReference.length()" because "bytes" is null

This PR avoids parsing the Workflow object if it doesn't exist.

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[sbcd90]

the workflow object is already nullable in kotlin. so, we do not need to use java.util.Optional here. https://typealias.com/guides/java-optionals-and-kotlin-nulls/
can we adjust this?