tui: fix search domain parsing from runtime environment info

tui: fix search domain parsing from runtime environment info

Commit 55dc67c ("tui: fix FQDN validation") mostly fixed FQDN
validation, but missed a case when the search domain only has one
component.

As Fqdn::from() requires at least two components to pass validation, the
search domain parsing from the runtime environment info provided by the
low-level installer failed. This resulted in that the network dialog
defaulted to "<product>.example.invalid" as FQDN, instead of
"<product>.<searchdomain>" as it should.

Fixes: 55dc67c ("tui: fix FQDN validation")
Reported-by: Aaron Lauterer <a.lauterer@proxmox.com>
Signed-off-by: Christoph Heiss <c.heiss@proxmox.com>