GCE: ex_create_firewall: allow creation og firewalls with sourceRanges = [], preserve behavior for sourceRanges = None, preserve default value.

GCE documentation states that firewall allows traffic
if it matches either sourceRanges or sourceTags values.
Thus, sourceRanges = [] is a valid parameter value.

Although it would be better to simply set ["0.0.0.0/0"]
as the default, for minimal API breakage, None value is
still supported.