There is no way to set the "blank" Unimarc 100 field $a/17-19 ... unused positions being filled with blanks.
Created attachment 177607 [details] unimarc_field_100.pl - Character positions 17 - 19
There is no way to use "x - Not applicable".
Join https://bugs.koha-community.org/bugzilla3/show_bug.cgi?id=29731