Fix Acl.getAccessListEntries() NPE

Fix Acl.getAccessListEntries() NPE

This reworks interactions with Acl.getAccessListEntries(), so that
it's non-presence is uniformly treated in all cases. This fixes at
least:

java.lang.NullPointerException: null
	at org.opendaylight.netvirt.aclservice.listeners.AclEventListener.remove(AclEventListener.java:110) ~[?:?]
	at org.opendaylight.netvirt.aclservice.listeners.AclEventListener.remove(AclEventListener.java:50) ~[?:?]
	at org.opendaylight.genius.datastoreutils.AsyncDataTreeChangeListenerBase$DataTreeChangeHandler.run(AsyncDataTreeChangeListenerBase.java:158) ~[274:org.opendaylight.genius.mdsalutil-api:0.7.1]
	at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149) ~[?:?]
	at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624) ~[?:?]
	at java.lang.Thread.run(Thread.java:748) [?:?]

While also consilading duplicate code.

JIRA: NETVIRT-1636
Change-Id: Ifc33c63aa41c0fb75453deeee2093906c2ad5e95
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>
(cherry picked from commit 6a1bd2bd02d4612c8f468d179d5cb0dc795f1d84)