modernc.org/sqlite 修复 DSN 别名选择逻辑

dsnPick treated an empty value as absent, so "_foreign_keys=on&_fk=" fell back to the primary key and enabled foreign keys. mattn/go-sqlite3 selects between a key and its alias by presence alone and then reads that key's value, so the empty alias wins and suppresses the PRAGMA entirely. Match that: pick on presence, return the selected key's value as-is. An empty value still skips the PRAGMA at the call sites, and skips validation with it, so an empty shorthand is not an error. Co-Authored-By: Claude Opus 4.8 <[email protected]>