VXGW: Fix IP handling in UcastMacsRemote table

This patch fixes Ip handling when searching for entries
in the Ucast_Macs_Remote table of the VTEP (in particular
the VTEP may return either null or an empty string when
the field is not filled).

This is needed for ARP supression (MN-1636)

NOTE: similar issues may affect the other tables...
(but should be addressed in a different patch)

Ref: MN-1636

Signed-off-by: Ernest Artiaga <ernest@midokura.com>