Support Null Safety

[GabrielAraujo]

Hey, I would like to help update this project to the most recent dependencies and add support to null safety.

I've just sent you an email so we can chat further if possible

[GabrielAraujo]

@vishna take a look at this forked project https://github.com/SpectoraSoftware/flutter_cache_manager_hive
Maybe it is worth it to bring the changes over an create e new version to pub.dev

[VB10]

@GabrielAraujo do you wanna create a new library or if you not have time, i can make it

[dev-intellisoft]

Any success? I am facing the same error while trying to use with news flutter/dart version.