fix: customize model credentials were invalid despite the provider cr…

…edentials being active (langgenius#8864)

api/core/entities/provider_configuration.py

@@ -119,7 +119,7 @@ def get_current_credentials(self, model_type: ModelType, model: str) -> Optional
                         credentials = model_configuration.credentials
                         break
 
-            if self.custom_configuration.provider:
+            if not credentials and self.custom_configuration.provider:
                 credentials = self.custom_configuration.provider.credentials
 
             return credentials