NCBC-697: Ensure ConfigObservers collection is not null

In certain situations when EnableConfigHeartbeat is enabled, the timer may
go off when the ConfigObservers collection is still null. This patch
ensures that collection is not null before trying to acquire the observer.

Change-Id: I4ba48e051d775a675563da1cb764edd82f283b32
Reviewed-on: http://review.couchbase.org/42377
Reviewed-by: Simon Baslé <simon@couchbase.com>
Reviewed-by: Jeffry Morris <jeffrymorris@gmail.com>
Tested-by: Jeffry Morris <jeffrymorris@gmail.com>