Elan interface add throwing NPE

Elan interface add throwing NPE

Issue Description:
=================
Elan instance name may be null, add to ConcurrentHashMap must throw NPE

ERROR | opendaylight-cluster-data-notification-dispatcher-40 |
DataTreeChangeListenerActor      | 235 -
org.opendaylight.controller.sal-clustering-commons - 1.9.1 |
member-2-shard-default-config: Error notifying listener
org.opendaylight.genius.mdsalutil.cache.DataObjectCache$$Lambda$1291/872119028@32410eae
java.lang.NullPointerException: null
    at org.opendaylight.netvirt.elan.cache.ElanInterfaceCache.added(ElanInterfaceCache.java:75)
    at org.opendaylight.netvirt.elan.cache.ElanInterfaceCache.added(ElanInterfaceCache.java:37)

Solution:
=========
Determine that the Elan instance name cannot be null before adding

Change-Id: I21f2f93427aec92b708725dd41aa37cd37e7dcac
Signed-off-by: zhangninglc <zhangninglc@inspur.com>