Handle nil cell value in compiled printer functions.

Handle nil cell value in compiled printer functions.

* doc/misc/ses.texi (Quick Tutorial): Minor clarification about the
ses-range `!' modifier.
(More on cell printing): Fix this that the fallback printer is
`ses-prin1', not "%S".  That makes a difference for any cell value for
which "%S" would insert a backslash characters.

* lisp/ses.el (ses-local-printer-compile): Handle the nil cell value
--- contrary to emacs-25 branches ses-call-printer does not handle
prior to calling a function printer.  Not doing this would still work
because the compiled function would throw and error and SES would in
the end resort to the ses-prin1 fallback, however this way would not
be in line with the raison d'être of compiling printer which is speed.