A database sometimes needs to model this case: the value is unknown: maybe the paper file you digitized was corrupted or destroyed, or other valid reasons this value is not known.
Whereas null references can be avoided, like in rust etc.
Rust or functional languages show you do not generally need null references, you have options instead. It is completely different.
Null in C or C++ is much more a very costly misstake. Missing data is just reality.
Data may be incomplete, the values unknown. SQL is designed for that. It is designed to handle reality. Whereas null references are not, they cause problems you do not need...
Data != reference
you can avoid the issue of allowing invalid references everywhere
you can not avoid incomplete data.