DData: if lmdb.dir is null or empty, log a warning and set to default

[Aaronontheweb]

Changes

DData: if lmdb.dir is null or empty, log a warning and set to default. Should address some NullReferenceException issues that occured when the following HOCON was set:

akka.cluster.distributed-data.durable.lmdb.dir = "shard-data"

Reported Exception:

[15:53:33 ERR] Stopping distributed-data Replicator due to load or startup failure in durable store, caused by: Exception during creation
[akka://cluster-system/system/sharding/replicator/durableStore#713307088]: Akka.Actor.ActorInitializationException: Exception during creation
 ---> System.NullReferenceException: Object reference not set to an instance of an object.
   at Akka.DistributedData.LightningDB.LmdbDurableStore.PreStart()
   at Akka.Actor.ActorBase.AroundPreStart()
   at Akka.Actor.ActorCell.<>cDisplayClass175_0.<Create>b0()
   at Akka.Actor.ActorCell.UseThreadContext(Action action)
   at Akka.Actor.ActorCell.Create(Exception failure)
   --- End of inner exception stack trace -

Checklist

For significant changes, please ensure that the following have been completed (delete if not relevant):

• This change follows the Akka.NET API Compatibility Guidelines.
• This change follows the Akka.NET Wire Compatibility Guidelines.
• I have reviewed my own pull request.

[Aaronontheweb]

Detailed my changes

[Aaronontheweb]

Resolve AK1004 warning

[Aaronontheweb]

Do quick null check and the log the warning so the user knows that there's something off with their configuration, but then make the system work using its normal defaults afterwards

[Arkatufus]

LGTM