Blender Git Loki

Fix T51176: Cache file location can be blank and prevent fluid simulation from reading baked data

Sanitize a bit how cache path is handled by fluidsim (there is much more
to be done here though :( ), and forbid empty path (we reset to default
path relative to current .blend file in case it's empty).

If people really, really want to use current OS-wise directory, they can at
least use '.' as path. ;)