fix get embedding model provider in empty dataset (#1986)

Co-authored-by: jyong <jyong@dify.ai>

api/core/model_manager.py

@@ -179,6 +179,8 @@ class ModelManager:
         :param model: model name
         :return:
         """
+        if not provider:
+            return self.get_default_model_instance(tenant_id, model_type)
         provider_model_bundle = self._provider_manager.get_provider_model_bundle(
             tenant_id=tenant_id,
             provider=provider,