labs: make wait_for_file work with nounset

The wait_for_file function (used for debugging) can be called with or
without an argument. Called without an argument, it will trigger an
error if the nounset option is on (set -o nounset).

This problem is fixed by adding a default value (empty string) to the
function argument.

Change-Id: I5beebc9cbf8ab62678ab2a557b36b359f5c1f9e4
Merge "labs: make wait_for_file work with nounset"