Fix nullptr regression in RegistryUtil::ReadRegistry

Fix nullptr regression in RegistryUtil::ReadRegistry

Merging PR #2057 caused the Windows build to fail:

shell_integration\windows\OCUtil\RegistryUtil.cpp(43): error C2664: 'LSTATUS RegOpenKeyExW(HKEY,LPCWSTR,DWORD,REGSAM,PHKEY)': cannot convert argument 3 from 'nullptr' to 'DWORD'

The previous implementation prior the PR supplied NULL as the argument 3 to RegOpenKeyEx,
so it was silently accepted and translated to zero, satisfying the DWORD's type requirement.

Signed-off-by: Michael Schuster <michael@schuster.ms>