In loopback 4, Hasmany relation returns undefined instead of empty array when there is no related data

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## Steps to reproduce

Query data by including hasmany relation..
```ts
// Exemple
result = await this.authorRepository.findOne({ where: { id : ''}, include:['books'] })
```
## Current Behavior

in the result, you will notice that books is undefined for authors who dont have any books.
```yaml
[
{
id : 'xxxx',
name : 'abc',
# books is undefined
},
{
id : 'xxxx',
name : 'abc',
books : [
{
id : 'xxxxx',
title : 'zzzzz'
}
]
}
]
```

## Expected Behavior

Books should return empty array for authors who don't have books. it was like that in loopback 3.
```yaml
[
{
id : 'xxxx',
name : 'abc',
books : [] #should be like that
},
{
id : 'xxxx',
name : 'abc',
books : [
{
id : 'xxxxx',
title : 'zzzzz'
}
]
}
]
```

## Link to reproduction sandbox

## Additional information

## Related Issues

_See [Reporting Issues](http://loopback.io/doc/en/contrib/Reporting-issues.html) for more tips on writing good issues_

Start with the authorRepository.findOne call using include: ['books'] and reproduce it with a MySQL relation where no related books exist. Trace how the hasMany include result is assembled, then verify that authors without related rows receive books: [] while authors with books retain their related records.

Written by the indexing model from the issue text.