BLOCKING ISSUE: Error with slice of map[string]interface{} during generation

### Reproduction repo
https://github.com/frederikhors/gqlgenIssueSliceMapStringInterface.

### Reproduction steps

1. `go run github.com/99designs/gqlgen init`

1. Add these lines to the default `schema.graphqls` file:

```graphql
input NewTodo {
text: String!
userId: String!
rows: [TodoRowInput!]
}

input TodoRowInput {
description: String
}
```

1. add these lines to `gqlgen.yml`:

```yml
TodoRowInput:
model: "map[string]interface{}"
```

1. At the top of `resolver.go` file, between package and import, add the following line:

`//go:generate go run github.com/99designs/gqlgen`

1. Run `go generate ./...`

### The error

```
validation failed: packages.Load: C:\gqlgenIssueSliceMapStringInterface\graph\generated\generated.go:1793:19: cannot use ec.unmarshalOTodoRowInput2mapᚄ(ctx, v) (value of type map[string]interface{}) as []map[string]interface{} value in assignment
exit status 1
graph\resolver.go:1: running "go": exit status 1
```

**Is this a bug?**