After setting a default dns_domain on a network
openstack net set --dns-domain 'example.com.' <NETWORKID>
the setting could not be reverted back to an empty string using
openstack net set --dns-domain '' <NETWORKID>
and the call also does not emit any error.
The same is true for the description of a network.
Reason was using the parsed argument directly as a condition instead of
comparing against None -- dropping the empty string as valid value.
The name parameter already accepted an empty string.
This change also adds a testcase for dns_domain, description and the
network name parameter, checking if the empty string is forwarded.
Change-Id: Ia7b9738205002b028c19e4f397411c86469cba1a
ce0765facb