[Bleeding] Account for null in EntityDamageEvent handling. Fixes BUKKIT-5317

Damage caused by explosions will return null for the event as of
6588d6f72bbca74bf150de65593ac575b846111b. As such, a null check is
now necessary when handling non-living entity damage events.

src/main/java/org/bukkit/craftbukkit/event/CraftEventFactory.java

@@ -455,8 +455,10 @@ public class CraftEventFactory {
         if (!(source instanceof EntityDamageSource)) {
             return false;
         }
-        // We don't need to check for null, since EntityDamageSource will always return an event
         EntityDamageEvent event = handleEntityDamageEvent(entity, source, damage);
+        if (event == null) {
+            return false;
+        }
         return event.isCancelled() || event.getDamage() == 0;
     }