[grid] Update DefaultSlotSelector

Author: pujagani

java/server/src/org/openqa/selenium/grid/distributor/selector/DefaultSlotSelector.java

@@ -51,7 +51,7 @@ public Set<SlotId> selectSlot(Capabilities capabilities, Set<NodeStatus> nodes)
           // And use the node id as a tie-breaker.
           .thenComparing(NodeStatus::getId))
       .flatMap(node -> node.getSlots().stream()
-        .filter(slot -> !slot.getSession().isPresent())
+        .filter(slot -> slot.getSession()==null)
         .filter(slot -> slot.isSupporting(capabilities))
         .map(Slot::getId))
       .collect(toImmutableSet());