Make TypeVisitor result type nullable

[pelepelin]

TypeVisitor is expected to return GraphQLNamedType | null | undefined in the code here
https://github.com/apollographql/graphql-tools/blob/9bfb11c51e2ca57579a950bd8de65adb0e1f37a3/src/transforms/visitSchema.ts#L60

However, it is declared as non-nullable here
https://github.com/apollographql/graphql-tools/blob/9bfb11c51e2ca57579a950bd8de65adb0e1f37a3/src/transforms/visitSchema.ts#L33-L36

Which leads to necessity of type cast when a null-returning visitor is defined.

TODO:

• [ ] If this PR is a new feature, reference an issue where a consensus about the design was reached (not necessary for small changes)
• [ ] Make sure all of the significant new logic is covered by tests
• Rebase your changes on master so that they can be merged easily
• Make sure all tests and linter rules pass
• Update CHANGELOG.md with your change. Include a description of your change, link to PR (always) and issue (if applicable). Add your CHANGELOG entry under vNEXT. Do not create a new version number for your change yourself.

[apollo-cla]

@pelepelin: Thank you for submitting a pull request! Before we can merge it, you'll need to sign the Apollo Contributor License Agreement here: https://contribute.apollographql.com/

[yaacovCR]

Closed in favor of #1307