resolve: refuse to resolve empty hostname

Previously, varlink or dbus methods return
io.systemd.Resolve.NoNameServers or BUS_ERROR_NO_NAME_SERVERS if an
empty hostname is provided, and thus nss-resolve returns NSS_STATUS_TRYAGAIN.

That causes getaddrinfo() returns 'Temporary failure in name resolution'
instead of 'Name or service not known'.

This makes calling varlink or dbus method with an empty hostname result
-EINVAL, and hence nss-resolve returns NSS_STATUS_NOTFOUND.

Fixes RHBZ#2039854 (https://bugzilla.redhat.com/show_bug.cgi?id=2039854).

src/shared/dns-domain.h

@@ -60,6 +60,10 @@ static inline int dns_name_is_valid_ldh(const char *s) {
         return 1;
 }
 
+static inline bool dns_name_is_empty(const char *s) {
+        return isempty(s) || streq(s, ".");
+}
+
 void dns_name_hash_func(const char *s, struct siphash *state);
 int dns_name_compare_func(const char *a, const char *b);
 extern const struct hash_ops dns_name_hash_ops;