The documentation in the above URL states:

auto_whitelist_path /path/to/file (default: ~/.spamassassin/auto-whitelist)
Automatic-whitelist directory or file.

Setting this configuration variable to a directory, as it states is possible, 
seems to not work.  The program seems to either try to open the directory as if 
it's a file, or try to create a file in the folder that has a zero-length file 
name (depending on whether or not the variable is terminated with a "/").

This behaviour is exhibited with the "spamassassin" executable, but probably 
also with "spamd" since spamd doesn't create any files in the directory when it 
is configured as a directory (and therefore auto-whitelisting does not work).

However, the "spamassassin" executable (and possibly also spamd) can still 
create the lock file, because it appends some characters to the end of the path 
to create a valid filename that either appears in the directory of the 
specified directory, or within the specified directory (again, depending on if 
a terminating slash is specified).

The bayes_path configuration variable seems to exhibit the same behaviour.  
However, bayes and autolearning still happen to work, because (as far as I can 
tell) every filename has some characters appended to it (_toks and _seen).  But 
you should check to make sure that it doesn't also try to create some file that 
has no appended characters.  If that is the case, then this same bug also 
applies to bayes_path.  (In my setup I'm setting bayes_path to be a filename, 
just in case).

Users can still use the program if they conclude that the documentation is 
wrong and specifying a directory will not work, and instead specify a 
filename.  But the documentation, or the program, should be changed so that the 
behaviour is as described.