Allow wildcard (empty) hostname GWAPI Routes

[rainest]

From #4512 (comment)

HTTPRoute and GRPCRoute translators drop Routes with no Hostname values. However, this field is optional, and there is no language indicating that empty Hostname Routes should be discarded. Listener documentation around Hostname indicates that empty is the universal wildcard, so I'd expect that applies to Routes also:

As a special case, one Listener within a group may omit Hostname, in which case this Listener matches when no other Listener matches.

Kong does support empty hostname routes and matches requests to them regardless of hostname.

generateKongRoutesFromHTTPRouteRule via ingressRulesFromHTTPRouteLegacyFallback would generate any-hostname rules, as it's just collecting the list of hostnames (which may be empty) before iterating over the rules.

Comparatively, generateKongRoutesFromHTTPRouteMatches via ingressRulesFromHTTPRouteWithCombinedServiceRoutes

I did not review ATC in depth, but I believe it has the same restriction (GRPCRoute did when I reviewed a related PR).

Although legacy handling does appear to allow empty hostnames, this restriction appears to be fairly old, however, so it's not quite clear how we ended up with that discrepancy. Regardless, we should allow empty hostname Routes.

Acceptance:

• KIC accepts HTTPRoutes and GRPCRoutes with no Hostname value.
• KIC generates Kong routes with no hostname from the rules of HTTPRoutes and GRPCRoutes with no Hostname value.