[hotfix][tests] Fix flaky NPE in SpeculativeExecutionTest

[rkhachatryan]

testCancelOtherScheduledCurrentExecutionsWhenAnyExecutionFinished starts scheduling asynchronously (createSchedulerAndStartScheduling(false)) and polls for the execution vertex via waitUntilIgnoringExceptions. That poll loop only guards against exceptions thrown while fetching the vertex; it discards the fetched value and unconditionally returns true, then the vertex is fetched a second, independent time right after the loop. Between these two fetches, the scheduler thread can still be populating the execution graph, so the second fetch occasionally observes a null element instead of throwing, causing:

NullPointerException: Cannot invoke
"ExecutionVertex.getCurrentExecutionAttempt()" because "ev" is null

Capture the vertex from the successful poll iteration itself, and make the poll condition require a non-null result, instead of re-querying the scheduler afterwards.

[flinkbot]

CI report:

• b2aec64 UNKNOWN

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