C#: Return an empty string if Device or Channel don't have a name

Signed-off-by: Paul Cercueil <paul.cercueil@analog.com>
Fixed non-working cyclic mode on devices using the low-speed interface

This fixes commit 4c6580b2, which has been pushed probably a bit too
fast.

Signed-off-by: Paul Cercueil <paul.cercueil@analog.com>