As my other comment points out, this is probably systems talking to systems talking to systems talking to systems to the nth degree. So even if the first system did in fact distinguish the NULL case from the string case, it only takes one system in the chain to be incapable of representing the difference to permanently and unrecoverably wreck it for all downstream systems.

What are the odds at least one system silently filters out apostrophes as invalid characters in license plate fields? Pretty good. These systems are often unattended, unmonitored processes often maintained by people who either can't fix errors upstream, or don't even want to, so these conversions are often written extremely permissively, trying to get through the data with whatever heuristics are necessary for the process to just Keep Working.