In Unicode it's named "NULL", line one of UnicodeData.txt[1] is `0000;<control>;Cc;0;BN;;;;;N;NULL;;;;`. Unlike ASCII characters Unicode code point names can be more than 3 letters.
Bytes are not strictly ASCII. Even within the realm of ASCII, if you have a binary with strings terminated in 0x00, those aren't generally called "NUL-terminated strings".
