ext/session: only return false when could not encode session at all#21181

Girgias wants to merge 2 commits intophp:masterphp/php-src:masterfrom

Girgias:2026-02-session-if-encode-null-then-failureGirgias/php-src:2026-02-session-if-encode-null-then-failureCopy head branch name to clipboard