run env: use default error message if country detection failed with empty string

run env: use default error message if country detection failed with empty string

Bit of perl fun again.
$err from detect_country_tracing_to() can be empty string under certain
circumstances (according to a forum post [0]). The // operator
evaluates an empty as true, thus `warn` receives an empty string to and
just prints

  Warning: something wrong at /usr/share/perl5/proxmox/Install/RunEnv.pm line 305

Which isn't particular helpful. Use the || operator instead, that
evaluates an empty string as false and thus would fall back to the
generic error message.

A minimal reproducer/example for completeness sake:

  #!/usr/bin/env perl
  use strict;
  use warnings;

  warn ('' // "unable to detect country\n");
  warn ('' || "unable to detect country\n");

gives

  Warning: something's wrong at ./test.pl line 5.
  unable to detect country

[0] https://forum.proxmox.com/threads/blank-screen-while-installing.143928/

Signed-off-by: Christoph Heiss <c.heiss@proxmox.com>