Fix NPE in BMP

Fix NPE in BMP

Empty lists are being squashed to nulls now, make sure we account
for this this possiblity. Fixes the following splat:

java.lang.NullPointerException: null
	at org.opendaylight.protocol.bmp.impl.app.BmpRouterPeerImpl.setPeerTables(BmpRouterPeerImpl.java:262) ~[bundleFile:?]
	at org.opendaylight.protocol.bmp.impl.app.BmpRouterPeerImpl.<init>(BmpRouterPeerImpl.java:145) ~[bundleFile:?]
	at org.opendaylight.protocol.bmp.impl.app.BmpRouterPeerImpl.createRouterPeer(BmpRouterPeerImpl.java:168) ~[bundleFile:?]
	at org.opendaylight.protocol.bmp.impl.app.BmpRouterImpl.onPeerUp(BmpRouterImpl.java:249) ~[bundleFile:?]
	at org.opendaylight.protocol.bmp.impl.app.BmpRouterImpl.onMessage(BmpRouterImpl.java:122) ~[bundleFile:?]
	at org.opendaylight.protocol.bmp.impl.session.BmpSessionImpl.handleMessage(BmpSessionImpl.java:127) ~[bundleFile:?]
	at org.opendaylight.protocol.bmp.impl.session.BmpSessionImpl.channelRead0(BmpSessionImpl.java:50) ~[bundleFile:?]
	at org.opendaylight.protocol.bmp.impl.session.BmpSessionImpl.channelRead0(BmpSessionImpl.java:33) ~[bundleFile:?]

Change-Id: I303095aa30f296f0bb77d4d11b6a31ee77494abb
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>