Bump github.com/guregu/null
Bumps github.com/guregu/null from 3.5.0+incompatible to 4.0.0+incompatible.
updated-dependencies:
- dependency-name: github.com/guregu/null dependency-type: direct:production update-type: version-update:semver-major
...
Signed-off-by: dependabot[bot] <support@github.com>