gluster: fixed NPE in GlusterAuditLogUtil

Modified methods logVolumeMessage() and logServerMessage()
to pass empty map for customValues to logAuditMessage()
instead of null, as it was causing an NPE in
AuditLogDirector#getAvailableValues()

Change-Id: Id3b563573670c379e085b641174cd7b1f0de2864
Signed-off-by: Shireesh Anjal <sanjal@redhat.com>