Fix NULL pointer crash in send_at_cmd

If the string argument of send_at_cmd is a null pointer,
then the function should not crash: indeed this is a valid
parameters which can be used to send an AT cmd which doesn't
require any extra parameter.

Test: trivial fix. Existing unit tests still pass.
Change-Id: I3d83eebed660d703cd5a93a19dc73f4f354ab7bf
Signed-off-by: Sebastien Griffoul <sebastien.griffoul@intel.com>