Fix OVSBridge.get_port_ofport to handle empty output

Previously ovs_lib.OVSBridge.get_port_ofport() was not handling the
case where db_get_val() was returning None when no output was recieved
from ovs-vsctl.  This patch ensures that the TypeError that results
from this condition is appropriately handled.

Change-Id: I168eb0f2c6f0cb98a3eb3e118972bbaf0383ff89
Partial-bug: #1336172
Merge "Fix OVSBridge.get_port_ofport to handle empty output"