Fix zero-valued metrics in event:page breakdown responses

[PepperCat-YamanekoVillage]

Fix zero-valued metrics in event:page breakdown responses

Summary

This PR fixes an issue where the event:page breakdown endpoint returned page entries with all metric values set to 0, despite aggregate statistics showing non-zero values for the same period.

Problem

Aggregate statistics were reported correctly, but page-level breakdowns using property=event:page returned rows with zero-valued metrics.

For example:

Aggregate request:

GET /api/v1/stats/aggregate?site_id=<site>&period=today&metrics=visitors,pageviews,visits,events

Aggregate response:

{
  "events": 0,
  "pageviews": 165,
  "visitors": 122,
  "visits": 132
}

Breakdown request:

GET /api/v1/stats/breakdown?site_id=<site>&period=today&property=event:page&metrics=visitors,pageviews,events&limit=20

Breakdown response:

{
  "events": 0,
  "page": "/articles/selfhosted/",
  "pageviews": 0,
  "visitors": 0
}

The issue was reproducible not only with period=custom, but also with other periods such as today and yesterday.