Migrating to null safety

[hewcaw]

I got dependency conflict between shelf and notion_api, can you migrate vercel-dart to null safety and also aws-lambda-dart-runtime is already supported null safety. Btw the process is pretty simple though, check out https://dart.dev/null-safety/migration-guide.

I tried to do it myself but failed pathetically, I don't know how to test the project locally with vercel dev so I published it to npmjs.com but it's still didn't resolve the conflict, maybe a trouble at build.sh > reading and writing pubspec.yam, make it still use the old shelf version (^0.7.9) instead of the new one I just added (^1.2.0)

Because every version of vercel_dart from git depends on shelf ^0.7.9 and api depends on shelf ^1.2.0, vercel_dart from git is forbidden.
So, because api depends on vercel_dart from git, version solving failed.