Don't fake empty cells value by "" when printing with a lambda.

Don't fake empty cells value by "" when printing with a lambda.

When using a lambda expression printer function the user should be
free to format differently a really empty cell, ie. containing nil,
from a cell containing an empty string "".

* ses.el (ses-call-printer): Replace `(or value "")' by just `value'
in the case of a lambda expression printer function.

* ses.texi (Printer functions): Add example and description about
lambda expression printer function handling all the possible values,
including unexpected ones.