I assume that they first tried /dev/null which failed, so then moved onto just plain null?
Otherwise it would not make sense that a unix programmer did this. More likely ula dos programmer misspelled NUL as null.
That's been a feature since DOS 2.0, there was even an undocumented option AVAILDEV to make the prefix mandatory, instead of having device names present everywhere. But it broke the common trick used to detect if a directory exists ("if exist c:\some\path\nul").