Fix test about need for explicit printing inserted lines

Fix test about need for explicit printing inserted lines

* lisp/ses.el (ses--blank-line-needs-printing-p): New
function. Does not consider that printer `nil' produces a non
empty string, as `ses-print-cell' removes nil printer by oring to
fallback.
(ses-insert-row): Replace the complex and erroneous test about
blank newline needing printing by a call to
'ses--blank-line-needs-printing-p'.