run env: do not store emtpy hostname

run env: do not store emtpy hostname

without this patch the hostname ends up as the empty string in
run-env-info.json, which results in a parse-error in the TUI code
(an empty string is not None, but still too short as hostname)

Minimally tested on a VM.

Fixes: bda1cdf ("run env: retrieve and store hostname from DHCP lease
 if available")
Signed-off-by: Stoiko Ivanov <s.ivanov@proxmox.com>