PlainTextFormatter: toString on null object reference

[rfc2822]

Random crashes like this:

EXCEPTION
java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String java.lang.Object.toString()' on a null object reference
	at at.bitfire.davdroid.log.PlainTextFormatter.format(PlainTextFormatter.kt:188)
	at at.bitfire.davdroid.log.LogcatHandler.publish(LogcatHandler.kt:18)
	at java.util.logging.Logger.log(Logger.java:738)
	at java.util.logging.Logger.doLog(Logger.java:765)
	at java.util.logging.Logger.log(Logger.java:831)
	at at.bitfire.davdroid.sync.SyncManager.performSync(SyncManager.kt:460)
	at at.bitfire.davdroid.sync.AddressBookSyncer.syncAddressBook(AddressBookSyncer.kt:152)
	at at.bitfire.davdroid.sync.AddressBookSyncer.syncCollection(AddressBookSyncer.kt:7)
	at at.bitfire.davdroid.sync.AddressBookSyncer.syncCollection(AddressBookSyncer.kt:1)
	at at.bitfire.davdroid.sync.Syncer.syncCollectionContents$davx5_404030003_4_4_3_gplayRelease(Syncer.kt:59)
	at at.bitfire.davdroid.sync.Syncer.sync$davx5_404030003_4_4_3_gplayRelease(Syncer.kt:36)
	at at.bitfire.davdroid.sync.Syncer.invoke(Syncer.kt:211)
	at at.bitfire.davdroid.sync.worker.BaseSyncWorker$doSyncWork$2.invokeSuspend$lambda$0(BaseSyncWorker.kt:1)
	at at.bitfire.davdroid.sync.worker.BaseSyncWorker$doSyncWork$2.$r8$lambda$MnrEU1_WtQZTTPjmMRztfG3Y7fo(BaseSyncWorker.kt:1)
	at at.bitfire.davdroid.sync.worker.BaseSyncWorker$doSyncWork$2$$ExternalSyntheticLambda0.invoke(R8$$SyntheticClass:3)
	at kotlinx.coroutines.InterruptibleKt$runInterruptible$2.invokeSuspend(Interruptible.kt:58)
	at kotlinx.coroutines.InterruptibleKt$runInterruptible$2.invoke(Interruptible.kt:13)
	at kotlinx.coroutines.intrinsics.UndispatchedKt.startUndispatchedOrReturn(Undispatched.kt:5)
	at kotlinx.coroutines.BuildersKt.withContext(Unknown Source:44)
	at kotlinx.coroutines.InterruptibleKt.runInterruptible$default(Interruptible.kt:9)
	at at.bitfire.davdroid.sync.worker.BaseSyncWorker$doSyncWork$2.invokeSuspend(BaseSyncWorker.kt:325)
	at kotlin.coroutines.jvm.internal.BaseContinuationImpl.resumeWith(ContinuationImpl.kt:9)
	at kotlinx.coroutines.DispatchedTask.run(DispatchedTask.kt:107)
	at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1145)
	at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:644)
	at java.lang.Thread.run(Thread.java:1012)

Seems to come from the PlainTextFormatter when format() is called with an argument that is null.