Bump github.com/guregu/null

Bumps [github.com/guregu/null](https://github.com/guregu/null) from 3.5.0+incompatible to 4.0.0+incompatible.
- [Release notes](https://github.com/guregu/null/releases)
- [Commits](https://github.com/guregu/null/compare/v3.5.0...v4.0.0)

---
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>

go.mod

@@ -10,7 +10,7 @@ require (
 	github.com/gorilla/mux v1.8.0
 	github.com/gorilla/schema v1.2.0
 	github.com/gorilla/sessions v1.2.1
-	github.com/guregu/null v3.5.0+incompatible
+	github.com/guregu/null v4.0.0+incompatible
 	github.com/hashicorp/go-multierror v1.1.1
 	github.com/ikeikeikeike/go-sitemap-generator/v2 v2.0.2
 	github.com/kylemcc/twitter-text-go v0.0.0-20180726194232-7f582f6736ec

go.sum

@@ -59,8 +59,8 @@ github.com/gorilla/securecookie v1.1.1 h1:miw7JPhV+b/lAHSXz4qd/nN9jRiAFV5FwjeKyC
 github.com/gorilla/securecookie v1.1.1/go.mod h1:ra0sb63/xPlUeL+yeDciTfxMRAA+MP+HVt/4epWDjd4=
 github.com/gorilla/sessions v1.2.1 h1:DHd3rPN5lE3Ts3D8rKkQ8x/0kqfeNmBAaiSi+o7FsgI=
 github.com/gorilla/sessions v1.2.1/go.mod h1:dk2InVEVJ0sfLlnXv9EAgkf6ecYs/i80K/zI+bUmuGM=
-github.com/guregu/null v3.5.0+incompatible h1:fSdvRTQtmBA4B4YDZXhLtxTIJZYuUxBFTTHS4B9djG4=
+github.com/guregu/null v4.0.0+incompatible h1:4zw0ckM7ECd6FNNddc3Fu4aty9nTlpkkzH7dPn4/4Gw=
-github.com/guregu/null v3.5.0+incompatible/go.mod h1:ePGpQaN9cw0tj45IR5E5ehMvsFlLlQZAkkOXZurJ3NM=
+github.com/guregu/null v4.0.0+incompatible/go.mod h1:ePGpQaN9cw0tj45IR5E5ehMvsFlLlQZAkkOXZurJ3NM=
 github.com/hashicorp/errwrap v1.0.0 h1:hLrqtEDnRye3+sgx6z4qVLNuviH3MR5aQ0ykNJa/UYA=
 github.com/hashicorp/errwrap v1.0.0/go.mod h1:YH+1FKiLXxHSkmPseP+kNlulaMuP3n2brvKWEqk/Jc4=
 github.com/hashicorp/go-multierror v1.0.0/go.mod h1:dHtQlpGsu+cZNNAkkCN/P3hoUDHhCYQXV3UM06sGGrk=