[ros-svn] [fireball] 19964: Respect the fact, that the driver can return NULL-pointer, instead of a string having 0 characters!

[fireball at svn.reactos.com]

Respect the fact, that the driver can return NULL-pointer, instead of a string having 0 characters!
(makes third-party drivers compatibility better).
This piece left from previous commit.


Updated files:
trunk/reactos/ntoskrnl/io/pnpmgr.c