Protect all SingleResultCallback observables from NPE

SCALA-555

Author: rozza

driver/src/it/scala/org/mongodb/scala/SmokeTestISpec.scala

@@ -49,6 +49,9 @@ class SmokeTestISpec extends RequiresMongoDBISpec with FuturesSpec {
       info("find first should return null if no documents")
       collection.find().first().futureValue shouldBe null // scalastyle:ignore
 
+      info("find should return an empty list")
+      collection.find().futureValue shouldBe empty
+
       info("Insert a document")
       collection.insertOne(document).futureValue should equal(Completed())
 

driver/src/main/scala/org/mongodb/scala/ObservableImplicits.scala

@@ -442,7 +442,10 @@ trait ObservableImplicits {
             subscription.request(1)
           }
 
-          override def onError(throwable: Throwable): Unit = completeWith("onError", () => observer.onError(throwable))
+          override def onError(throwable: Throwable): Unit = throwable match {
+            case npe: NullPointerException => onComplete()
+            case _                         => completeWith("onError", () => observer.onError(throwable))
+          }
 
           override def onComplete(): Unit = {
             completeWith("onComplete", { () =>