Fix compiling errors

Author: raman-m

src/Ocelot/Configuration/AuthenticationOptions.cs

@@ -39,7 +39,7 @@ public AuthenticationOptions(List<string> allowedScopes, string[] authentication
         /// <param name="keys">New <see cref="AuthenticationProviderKeys"/> to build.</param>
         private void BuildAuthenticationProviderKeys(string legacyKey, string[] keys)
         {
-            keys ??= new string[];
+            keys ??= Array.Empty<string>();
             if (string.IsNullOrEmpty(legacyKey))
             {
                 return;

src/Ocelot/Configuration/Builder/AuthenticationOptionsBuilder.cs

@@ -25,9 +25,9 @@ public AuthenticationOptionsBuilder WithRequiredRole(List<string> requiredRole)
 
         [Obsolete("Use the " + nameof(WithAuthenticationProviderKeys) + " property!")]
         public AuthenticationOptionsBuilder WithAuthenticationProviderKey(string authenticationProviderKey)
-            => WithAuthenticationProviderKeys([authenticationProviderKey]);
+            => WithAuthenticationProviderKeys(authenticationProviderKey);
 
-        public AuthenticationOptionsBuilder WithAuthenticationProviderKeys(string[] authenticationProviderKeys)
+        public AuthenticationOptionsBuilder WithAuthenticationProviderKeys(params string[] authenticationProviderKeys)
         {
             _authenticationProviderKeys = authenticationProviderKeys;
             return this;