Allow null last bookmark in session

So it is possible to "break" causal consistency within a session by
calling `#beginTransaction(null)`.

Author: lutovich

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

@@ -125,7 +125,7 @@ public synchronized void reset()
         if ( currentTransaction != null )
         {
             currentTransaction.markToClose();
-            setLastBookmark( currentTransaction.bookmark() );
+            updateLastBookmarkFrom( currentTransaction );
             currentTransaction = null;
         }
         if ( currentConnection != null )
@@ -177,7 +177,7 @@ public synchronized Transaction beginTransaction()
     @Override
     public synchronized Transaction beginTransaction( String bookmark )
     {
-        setLastBookmark( bookmark );
+        lastBookmark = bookmark;
         return beginTransaction();
     }
 
@@ -195,10 +195,7 @@ public <T> T writeTransaction( Function<Transaction,T> work )
 
     void setLastBookmark( String bookmark )
     {
-        if ( bookmark != null )
-        {
-            lastBookmark = bookmark;
-        }
+        lastBookmark = bookmark;
     }
 
     @Override
@@ -225,7 +222,7 @@ public synchronized void onTransactionClosed( ExplicitTransaction tx )
         if ( currentTransaction != null && currentTransaction == tx )
         {
             closeCurrentConnection();
-            setLastBookmark( currentTransaction.bookmark() );
+            updateLastBookmarkFrom( currentTransaction );
             currentTransaction = null;
         }
     }
@@ -376,6 +373,14 @@ private void closeCurrentConnection( boolean sync )
         }
     }
 
+    private void updateLastBookmarkFrom( ExplicitTransaction tx )
+    {
+        if ( tx.bookmark() != null )
+        {
+            lastBookmark = tx.bookmark();
+        }
+    }
+
     private static List<Throwable> recordError( Throwable error, List<Throwable> errors )
     {
         if ( errors == null )

driver/src/test/java/org/neo4j/driver/internal/NetworkSessionTest.java

@@ -601,14 +601,28 @@ public void setLastBookmark()
     }
 
     @Test
-    public void notPossibleToOverwriteBookmarkWithNull()
+    public void possibleToOverwriteBookmarkWithNull()
     {
         NetworkSession session = newSession( mock( ConnectionProvider.class ), WRITE );
         session.setLastBookmark( "TheBookmark" );
 
         session.setLastBookmark( null );
 
-        assertEquals( "TheBookmark", session.lastBookmark() );
+        assertNull( session.lastBookmark() );
+    }
+
+    @Test
+    public void allowsToStartTransactionWithNullBookmark()
+    {
+        ConnectionProvider connectionProvider = mock( ConnectionProvider.class );
+        PooledConnection connection = openConnectionMock();
+        when( connectionProvider.acquireConnection( READ ) ).thenReturn( connection );
+        NetworkSession session = newSession( connectionProvider, READ );
+        session.setLastBookmark( "SomeUndesiredBookmark" );
+
+        session.beginTransaction( null );
+
+        assertNull( session.lastBookmark() );
     }
 
     @Test