fix(deps): update module gopkg.in/guregu/null.v3 to v4

[renovate]

This PR contains the following updates:

Package	Type	Update	Change
gopkg.in/guregu/null.v3	require	major	v3.5.0 -> v4.0.0

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Release Notes

guregu/null (gopkg.in/guregu/null.v3)

v4.0.0: Major brush-up

Compare Source

This is a new major release that improves performance and fixes an inconsistency that existed in v3.

• null.Time and zero.Time now embed sql.NullTime.
• null.Time and zero.Time's UnmarshalText returns a blank string if invalid, making them consistent with the rest of the package (see #​47).
• Errors returned by this package are now wrapped using Go 1.13-style errors.
• Support for unmarshaling from the JSON representation of sql.NullXXX objects (ex. {"Int64": 123, "Valid": true}) has been dropped, significantly improving unmarshaling time and reducing allocations.
• Unmarshaling errors no longer affect Valid. Always check for errors.

Support for Equal was added in the previous release.

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