bugfix: fix mtp worker empty run issue for deepseek dp.

[phantomlei3]

No description provided.

[gemini-code-assist]

Code Review

This pull request addresses a bug related to handling empty runs in a data-parallel setup for MTP workers. The changes introduce necessary checks to gracefully handle cases where a worker receives no sequences to process. In mtp_worker_impl.cpp, a guard is added to update_input_by_last_step_output to exit early if there are no sequences or embedding IDs. Similarly, in worker_impl.cpp, step_async is updated to verify that there are tokens to process before attempting to update the input. These changes appear to be a correct and robust solution for the issue. I have reviewed the changes and found no issues of high or critical severity.