optimizes login method

PiwigoDotnet/Piwigo.Client/PiwigoContext.cs

@@ -22,21 +22,22 @@ public class PiwigoContext : IPiwigoContext
 
     public async Task LoginAsync()
     {
+        if (IsLoggedIn)
+        {
+            _logger.LogWarning("The client is already logged in!");
+            return;
+        }
+
         var userName = _config.UserName;
         if (string.IsNullOrWhiteSpace(userName))
         {
-            throw new ArgumentException("Value cannot be null or whitespace.", nameof(userName));
+            throw new PiwigoException("username cannot be null or whitespace.");
         }
 
         var password = _config.Password;
         if (string.IsNullOrWhiteSpace(password))
         {
-            throw new ArgumentException("Value cannot be null or whitespace.", nameof(password));
+            throw new PiwigoException("password cannot be null or whitespace.");
-        }
-
-        if (IsLoggedIn)
-        {
-            throw new PiwigoException("The client is already logged in. Create a new instance or log out first!");
         }
 
         _logger.LogInformation("Logging into {PiwigoBaseUri} using username {Username}", _config.BaseUri, userName);