fix(deps): update module gopkg.in/guregu/null.v4 to v6

[renovate]

This PR contains the following updates:

Package	Change	Age	Confidence
gopkg.in/guregu/null.v4	v4.0.0 → v6.0.0

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[renovate]

⚠️ Artifact update problem

Renovate failed to update an artifact related to this branch. You probably do not want to merge this PR as-is.

♻ Renovate will retry this branch, including artifacts, only when one of the following happens:

• any of the package files in this branch needs updating, or
• the branch becomes conflicted, or
• you click the rebase/retry checkbox if found above, or
• you rename this PR's title to start with "rebase!" to trigger it manually

The artifact failure details are included below:

File name: go.sum

Command failed: go get -t ./...
go: gopkg.in/guregu/null.v6@v6.0.0: parsing go.mod:
	module declares its path as: github.com/guregu/null/v6
	        but was required as: gopkg.in/guregu/null.v6