Change in Ovsdb-node OtherConfig is not reflected in Datastore when 'other_config' column is cleared from openvswitch table

When 'other_config' column is cleared from openvswitch table at OVS switch side, then openvswitch table update notification is not handled properly in the OVSDB southbound plugin.

setOtherConfig() of OVSDB southbound plugin does not take care the case when
OtherConfig is non-NULL for OpenVSwitch old entry and OtherConfig is NULL for OpenVSwitch new entry.