Handle nulls returned from async tx functions

Previously code failed with NPE when provided async transaction
function returned null.

This commit fixes the problem by wrapping null in a completed future.

driver/src/main/java/org/neo4j/driver/internal/NetworkSession.java

@@ -360,7 +360,10 @@ private <T> CompletionStage<T> safeExecuteWork( ExplicitTransaction tx, Transact
         // sync failure will result in an exception being thrown
         try
         {
-            return work.execute( tx );
+            CompletionStage<T> result = work.execute( tx );
+
+            // protect from given transaction function returning null
+            return result == null ? completedFuture( null ) : result;
         }
         catch ( Throwable workError )
         {

driver/src/test/java/org/neo4j/driver/v1/integration/SessionAsyncIT.java

@@ -1157,6 +1157,16 @@ public void shouldAllowAccessingRecordsAfterSessionClosed()
         }
     }
 
+    @Test
+    public void shouldAllowReturningNullFromAsyncTransactionFunction()
+    {
+        CompletionStage<Object> readResult = session.readTransactionAsync( tx -> null );
+        assertNull( await( readResult ) );
+
+        CompletionStage<Object> writeResult = session.writeTransactionAsync( tx -> null );
+        assertNull( await( writeResult ) );
+    }
+
     private Future<List<CompletionStage<Record>>> runNestedQueries( StatementResultCursor inputCursor )
     {
         CompletableFuture<List<CompletionStage<Record>>> resultFuture = new CompletableFuture<>();

driver/src/test/java/org/neo4j/driver/v1/integration/SessionIT.java

@@ -1529,6 +1529,16 @@ public void shouldAllowLongRunningQueryWithConnectTimeout() throws Exception
         }
     }
 
+    @Test
+    public void shouldAllowReturningNullFromTransactionFunction()
+    {
+        try ( Session session = neo4j.driver().session() )
+        {
+            assertNull( session.readTransaction( tx -> null ) );
+            assertNull( session.writeTransaction( tx -> null ) );
+        }
+    }
+
     private void assumeServerIs31OrLater()
     {
         ServerVersion serverVersion = ServerVersion.version( neo4j.driver() );