Migration to null-safety

[ju2p]

Hey, I'm currently creating an app using flutter 2 and dart null safety, 'm using your package but there is no null-safe version so I made one.

Giving it back to you depending on whether or not you want to provide a null-safe version.

(I also changed a small part from the example app so that there is no overflow if picked image is too big)

✌️