Incorrect KPath use in VFS where NULL paths are allowed

[bonefish]

There seem to be a few places in the VFS code (particularly the _kern_*() functions that accept FD and path combos) where instead of a NULL path an empty path buffer is passed to the worker functions, while a NULL path should be passed instead.

[axeld]

Wouldn't it make more sense to have KPath not allocate a buffer in case it was constructed with a NULL path by default, and therefore also return NULL as path itself?

That would simplify the code in the VFS quite a bit, at least.

[axeld]

Fixed all occasions in vfs.cpp in hrev51145 using the KPath::LAZY_ALLOC feature.