fix first sync check NPE

[jrhizor]

At the end of my successful sync with a workspace that was created before #7832, I get:

java.lang.NullPointerException: Cannot invoke "java.lang.Boolean.booleanValue()" because the return value of "io.airbyte.config.StandardWorkspace.getFirstCompletedSync()" is null
	at io.airbyte.scheduler.app.JobSubmitter.lambda$submitJob$2(JobSubmitter.java:147) ~[io.airbyte.airbyte-scheduler-app-0.32.2-alpha.jar:?]
	at io.airbyte.commons.concurrency.LifecycledCallable.onSuccess(LifecycledCallable.java:99) [io.airbyte-airbyte-commons-0.32.2-alpha.jar:?]
	at io.airbyte.commons.concurrency.LifecycledCallable.call(LifecycledCallable.java:79) [io.airbyte-airbyte-commons-0.32.2-alpha.jar:?]
	at java.util.concurrent.FutureTask.run(FutureTask.java:264) [?:?]
	at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1136) [?:?]
	at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:635) [?:?]
	at java.lang.Thread.run(Thread.java:833) [?:?]

This isn't that terrible since it doesn't actually break the sync.