MockStore find and filter fail if no items have ever been added for the type

[snstanton]

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Describe the bug

If the store does not have an entry for typeName yet, then filter() or find() of the type raises:
TypeError: Cannot convert undefined or null to object

To Reproduce Steps to reproduce the behavior:

Create a MockStore with no entries. Call store.filter('foo', x => true) and see the exception

Expected behavior

It should return an empty array instead of raising an exception.

Environment:

• OS: macos
• @graphql-tools/mock: 9.0.3
• NodeJS: v22.2.0

Additional context