fix: TS type error on `strictNullChecks: true (#7931)

`Partial<T>` includes `{}`, therefore `NewVisitor[keyof NewVisitor]` includes `undefined`, and indexing `undefined` is error.

Issue: #7519

.changeset/purple-eyes-fix.md

@@ -0,0 +1,29 @@
+---
+'@graphql-codegen/plugin-helpers': patch
+---
+
+Fix TS type error on strictNullChecks: true
+
+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 

packages/utils/plugins-helpers/src/oldVisit.ts

@@ -3,8 +3,8 @@ import { ASTNode, visit } from 'graphql';
 type VisitFn = typeof visit;
 type NewVisitor = Partial<Parameters<VisitFn>[1]>;
 type OldVisitor = {
-  enter?: Partial<Record<keyof NewVisitor, NewVisitor[keyof NewVisitor]['enter']>>;
-  leave?: Partial<Record<keyof NewVisitor, NewVisitor[keyof NewVisitor]['leave']>>;
+  enter?: Partial<Record<keyof NewVisitor, NonNullable<NewVisitor[keyof NewVisitor]>['enter']>>;
+  leave?: Partial<Record<keyof NewVisitor, NonNullable<NewVisitor[keyof NewVisitor]>['leave']>>;
 } & NewVisitor;
 
 export function oldVisit(