Fix multisession.el when SQLite becomes unavailable

Fix multisession.el when SQLite becomes unavailable

This is for the case when the user customizes multisession
to use SQLite, but then built-in SQLite support becomes
unavailable for some reason (e.g., upgrade the OS).
* lisp/emacs-lisp/multisession.el (multisession-backend-value)
(multisession--backend-set-value, multisession--backend-values)
(multisession--backend-delete): Call next method if built-in
SQLite support is not available.  Suggested by Stefan Monnier
<monnier@iro.umontreal.ca>.  (Bug#72788)

(cherry picked from commit e2154c63761add6063a719ba6253c8e6e8133814)