Fix TS type error on strictNullChecks: true

[dittos]

Description

Fix the compiler error:

node_modules/@graphql-codegen/plugin-helpers/oldVisit.d.ts:5:75 - error TS2339: Property 'enter' does not exist on type '{ readonly enter?: ASTVisitFn<NameNode> | undefined; readonly leave: ASTReducerFn<NameNode, unknown>; } | { readonly enter?: ASTVisitFn<DocumentNode> | undefined; readonly leave: ASTReducerFn<...>; } | ... 41 more ... | undefined'.

5     enter?: Partial<Record<keyof NewVisitor, NewVisitor[keyof NewVisitor]['enter']>>;
                                                                            ~~~~~~~

node_modules/@graphql-codegen/plugin-helpers/oldVisit.d.ts:6:75 - error TS2339: Property 'leave' does not exist on type '{ readonly enter?: ASTVisitFn<NameNode> | undefined; readonly leave: ASTReducerFn<NameNode, unknown>; } | { readonly enter?: ASTVisitFn<DocumentNode> | undefined; readonly leave: ASTReducerFn<...>; } | ... 41 more ... | undefined'.

6     leave?: Partial<Record<keyof NewVisitor, NewVisitor[keyof NewVisitor]['leave']>>;
                                                                            ~~~~~~~


Found 2 errors in the same file, starting at: node_modules/@graphql-codegen/plugin-helpers/oldVisit.d.ts:5

Only happens when TS compiler options strictNullChecks: true and skipLibCheck: false.

Partial<T> includes {}, therefore NewVisitor[keyof NewVisitor] includes undefined, and indexing undefined is error.
Eliminate undefined by wrapping it inside NonNullable<...>.

Related #7519

Type of change

• Bug fix (non-breaking change which fixes an issue)

How Has This Been Tested?

1. git clone https://github.com/dittos/graphql-codegen-ts-strict-repro.git
2. npm install
3. Apply the fix to node_modules/@graphql-codegen/plugin-helpers/oldVisit.d.ts
4. $(npm bin)/tsc

Test Environment:

• OS: Ubuntu 20.04.1 LTS
• @graphql-codegen/plugin-helpers: 2.4.2
• NodeJS: v16.13.2

Checklist:

• I have followed the CONTRIBUTING doc and the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules

[changeset-bot]

🦋 Changeset detected

Latest commit: eb0f852

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@graphql-codegen/plugin-helpers	Patch

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Name	Status	Preview	Updated
graphql-code-generator	✅ Ready (Inspect)	Visit Preview	Jul 6, 2022 at 0:57AM (UTC)

[n1ru4l]

@dittos Please create a changeset for the affected package as instructed via #7931 (comment)