fix hiding of signup link (#6113)

The registration link should be hidden if signup is not allowed and
whitelist is empty unless mail is disabled and invitations are allowed

src/api/core/mod.rs

@@ -225,7 +225,7 @@ fn config() -> Json<Value> {
           "url": "https://github.com/dani-garcia/vaultwarden"
         },
         "settings": {
-            "disableUserRegistration": !crate::CONFIG.signups_allowed() && crate::CONFIG.signups_domains_whitelist().is_empty(),
+            "disableUserRegistration": crate::CONFIG.is_signup_disabled()
         },
         "environment": {
           "vault": domain,