Bug 14709: Do not access to C4::Context->userenv from the commandline

Bug 14709: Do not access to C4::Context->userenv from the commandline

The script dies with
  Can't use an undefined value as a HASH reference at tools/export.pl
line 149.
if it is called from the command line.

This is introduced by bug 13040.

The C4::Context->userenv is not defined in this case.

Test plan:
Execute the script using the command line.
With the patch applies you should not get the error.

Signed-off-by: Joonas Kylmälä <j.kylmala@gmail.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Tomas Cohen Arazi <tomascohen@unc.edu.ar>