Fix incorrect path name check.

This was caught by clang 3.6's -Wpointer-bool-conversion. The device_path
field is a char[256], and thus never NULL. The intention for this code was
to check whether or not there is a named path at all. Checking whether this
is an empty string matches the original intent.

Change-Id: I5ccedc03167e6a457e472089de26130aff7f96e3