Unless you move or read something (SPACES, ZEROS, or the 1st eight chars of the Gettysburg Address) into WS-BIRTH-DATE-ZERO-CHECK the FILLER will contain whatever was in that piece of memory when it was allocated to the pgm at execution time. Its content can change from exec to exec. You are usually cautioned about making any judgements about its content.
I know CICS has an option to init GETMAINed data to binary zeros and I recall vaguely that some compilers (or is it systems?) format memory to binary zeros before making it available to pgms.