Add missing null return types in base-provider

[sebastinez]

Hey @ricmoo,

I created this PR to define null as return type in some functions in providers/src.ts/base-provider.ts since it didn't match with the functions implementations, where the functions return null.

It's the same issue that issue #1850 brought up.

Feel free to let me know of any feedback.

[ricmoo]

I can’t add this to v5 unfortunately. I have tried in the past and it breaks so many downstream dependencies.

In v5, there is no strict type checking in the TS config, so anything can return a null value. In v6, I am using strict which includes strictNullCheck, so this won’t be an issue.

So this won’t be an issue much longer. For now, keep in mind v5 does not strictly check null.

[ricmoo]

Closing this as it isn't compatible with v5 and v6 already has this in place.

Thanks! :)