Fixes population of joins that target relationship fields that have
`relationTo` as an array, for example:
```ts
// Posts collection
{
name: 'polymorphic',
type: 'relationship',
relationTo: ['categories', 'users'],
},
// Categories collection
{
name: 'polymorphic',
type: 'join',
collection: 'posts',
on: 'polymorphic',
}
```
Thanks @jaycetde for the integration test
https://github.com/payloadcms/payload/pull/12278!
---------
Co-authored-by: Jayce Pulsipher <jpulsipher@nav.com>
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