Use default config for sites which do not have a zone

[BlackbitDevs]

Q	A
Bug fix?	no
New feature?	yes
BC breaks?	not sure
Deprecations?	no

If you have configured zones as described in https://github.com/dachcom-digital/pimcore-i18n/blob/master/docs/20_Zones.md and afterwards add a new site /new-site, add localization pages like /new-site/de and assign it a language, you will get the error message:

This document can not be edited because it will be skipped during a i18n redirection or does not have a valid language property.
No i18n zone for domain "new-site.example.org" found. Maybe there is a typo in your i18n.zones.domains mapping?

Although

pimcore-i18n/src/Builder/ZoneBuilder.php

Lines 57 to 60 in c7381c1

	// no valid zone found. use default one.
	if ($validZone === false) {
	throw new \Exception(sprintf('No i18n zone for domain "%s" found. Maybe there is a typo in your i18n.zones.domains mapping?', $site->getMainDomain()));
	}

pretends // no valid zone found. use default one., it throws the exception which does not fall back to using default zone.

With this PR a temporary zone based on the default configuration will get created for the main domain of the site.