Enable `nilnil` linter

[silverwind]

It's bad practive to write functions in go that return no result but also no error, e.g. return value nil, nil because it leaves the reader confused as developers are used to the fact that if there is no error, then the return value is valid.

The nilnil linter can detect such scenarios and we should enable it.

The challenge with this is that there are many violations in the current code base. When I run with the strictest config, there are currently 287 violations.

nilnil:
  only-two: false
  detect-opposite: true

Maybe we could add the linter and // nolint:nilnil all places so that these can be gradually refactored.

[silverwind]

When running the linter with the default config (only 2 return values, no opposite check), it returns only 89 errors currently which may be more managable as a start.