[BUGFIX] Fix external redirects with custom pageNotFound handler

[SomeBdyElse]

Fix external redirects in multi-page, multi-domain setups with custom pageNotFound handlers.

Previously, external redirects for the non-default domain would fail. A configuration of the defaultRootPageId is not always an, as possible root pages may depend on the current domain. The TSFE would then be created for page uid 1 (last fallback). Page uid 1 is typically unavailable for domains other than the default domain. Therefore the fake TSFE is unable to determine its page id and output 404.

Defaulting the page uid 0 will allow the TSFE to determine the page id by domain records.