run command: use explicit return undef in closures on call sites

run command: use explicit return undef in closures on call sites

To avoid a misinterpretation of the auto-return value:

> In the absence of an explicit return, a subroutine, eval, or do FILE
> automatically returns the value of the last expression evaluated.
-- https://perldoc.perl.org/functions/return

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>