Per Page returns default when 0 is passed in for Search Pagination

On Agg calls, not interested in any of the hits items.

Example.
```
{
users(aggs:{
key:"my_agg",
value: {
terms: {field:firstName, size:1000}
}
},
perPage: 0)
{
aggregations
items {
...
}
}
}
```

The above will still return 20 items

Reproduce the users aggregation example with perPage: 0 and inspect the search pagination path that applies the default page size. Done means an aggregation-only request returns no hit items instead of the default 20, while the aggregations remain available; add or update a regression test if the repository has coverage for this path.

Written by the indexing model from the issue text.