MESSENGER-3746 Fix for mention pills

bwi/AppConfig/AppConfigService.swift

@@ -155,7 +155,7 @@ extension UserDefaults
         }
     }
     
-    func permalinkUrl() -> String {
+    func permalinkUrl() -> String? {
         if let url = appConfig.permalinkUrl {
             return "https://" + url
         }  else if let url = ServerURLHelper.shared.httpsPermalink() {
@@ -163,7 +163,7 @@ extension UserDefaults
         } else if let url = BuildSettings.clientPermalinkBaseUrl {
             return url
         } else {
-            return ""
+            return nil
         }
     }