fix(api): Fix the issues that `None` content is written to the user accidentally

The `_save_multimodal_output_and_convert_result_to_markdown` should not yield
anything when `contents` is None. However, the lack of `return` statement
causing the fallback branch executed, resulting in `None` written to the user.

api/core/workflow/nodes/llm/node.py

@@ -80,6 +80,7 @@ from extensions.ext_database import db
 from models.model import Conversation
 from models.provider import Provider, ProviderType
 from models.workflow import WorkflowNodeExecutionStatus
+
 from .entities import (
     LLMNodeChatModelMessage,
     LLMNodeCompletionModelPromptTemplate,