URQL GraphCache creates type error when there are no mutations

### Issue workflow progress

- [x] 1. The issue provides a reproduction: https://github.com/matthew-petrie/graphql-code-generator-issue-sandbox-template
- [x] 2. A failing test has been provided (see linked PR)
- [x] 3. A local solution has been provided (see https://github.com/matthew-petrie/graphql-code-generator-issue-sandbox-template)
- [x] 4. A pull request is pending review

**Describe the bug**

The `typescript-urql-graphcache` plugin causes a typescript error in the generated type file for the type `GraphCacheOptimisticUpdaters` if the schema has no mutations:

**To Reproduce**
Reproduction repo: https://github.com/matthew-petrie/graphql-code-generator-issue-sandbox-template

Steps to reproduce the behavior:

1. My GraphQL schema:

```graphql
type Query {
user(id: ID!): User!
}

type User {
id: ID!
username: String!
email: String!
}
```

2. My GraphQL operations:

```graphql
query user {
user(id: 1) {
id
username
email
}
}
```

3. My `codegen.yml` config file:

```yaml
schema: schema.graphql
documents: document.graphql
generates:
types.ts:
plugins:
- typescript
- typescript-operations
- typescript-urql-graphcache
```

**Expected behavior**

For `typescript-urql-graphcache` to generate a types file with no Typescript errors. If there are no mutations `GraphCacheOptimisticUpdaters` should be output as:
```graphql
export type GraphCacheOptimisticUpdaters = {};
```

**Environment:**

- OS: macOS Monterey 12.3 - M1 Pro chip
- `@graphql-codegen/typescript-urql-graphcache`: ^2.2.13
- `typescript`: ^4.7.2
- NodeJS: v16.13.1