KabylakeOpenBoardPkg/AspireVn7Dash572G/BoardEcLib: Check for NULL

Check that data pointers are not NULL and update the documented return
values. Also update some notes on this library.

Cc: Chasel Chiu <chasel.chiu@intel.com>
Cc: Nate DeSimone <nathaniel.l.desimone@intel.com>
Signed-off-by: Benjamin Doron <benjamin.doron00@gmail.com>
Reviewed-by: Chasel Chiu <chasel.chiu@intel.com>