Domain alias request alter fails when path prefix negotiation returns no match

Problem/Motivation

When the path prefix feature is enabled (domain.path_prefix parameter is TRUE), DomainAliasHooks::domainRequestAlter() calls $this->negotiator->negotiateByPathPrefix($candidates) to disambiguate domains sharing the same hostname. However, negotiateByPathPrefix() can return NULL when no candidate matches the current request path. In that case, $domain is set to NULL, the $domain instanceof DomainInterface check fails, and an error is logged:

Found matching alias %alias for host request %hostname, but no domain with a matching path prefix was found.

This is too harsh: the alias was correctly resolved to a valid target domain — path prefix disambiguation simply couldn't narrow it further. The behavior should gracefully fall back to the alias target domain, consistent with the non-prefix code path which unconditionally sets $domain = $target.

Steps to reproduce

1. Enable the path prefix feature.
2. Create two domains sharing a hostname, one with a path prefix and one without.
3. Create an alias pointing to one of these domains.
4. Visit a URL that matches the alias but whose path does not match any domain's prefix.
5. An error is logged instead of resolving to the target domain.

Proposed resolution

Add a null coalescing fallback so that when negotiateByPathPrefix() returns NULL, the alias target domain is used:

$domain = $this->negotiator->negotiateByPathPrefix($candidates) ?? $target;