sanitylib: Fix discard_report to check self.discards dict

Modifying discard_report function to fix the check for
self.discards dictionary in try block. self.discards
is None is dead code which didnot check if the dictionary was
empty. Changed it to if not self.discards.

Signed-off-by: Aastha Grover <aastha.grover@intel.com>