Page model api throws NPE for site without channel

Since v14.2 page model throws NPE for sites without channel configuration.
Cause is org.hippoecm.hst.pagemodelapi.v09.core.container.PageModelAggregationValve line #297

Reproduction steps would be:

• Create hst configuration without mapping to a channel in the channel manager
• Enable the page model API for this site
• Try to access http://localhost:8080/site/example/resourceapi

Results in server error and NPE in the logs.