OVSDB node ExternalIds update is not reflected when 'external_ids' column is cleared from openvswitch table.

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

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