ix bugs when a SES buffer has named cells, and another don't.

ix bugs when a SES buffer has named cells, and another don't.

* lisp/ses.el (ses-list-named-cells): Give a message when the list is empty.
(ses--unbind-cell-name): Do not throw error in a SES buffer w/o
any named cell for gethash from 'ses--named-cell-hashmap' being
'nil'.

(cherry picked from commit 2102554a486e457cf020a03b72276e532afc98ea)