ntdll: fix RtlCheckRegistryKey when called with parameter path being null or empty

[Stefan Leichter]

Found some corner cases when testing RtlCreateRegistryKey. As 
RtlCheckRegistryKey is working in the same area i decided to test this 
function too and fix it first.

Signed-off-by: Stefan Leichter <sle85276(a)gmx.de>
---
  dlls/ntdll/reg.c       |  5 +++++
  dlls/ntdll/tests/reg.c | 13 +++++++++++++
  2 files changed, 18 insertions(+)