Fix add_text/regenerate return tuple size mismatch in single-model server

[Chessing234]

Bug

Fixes #3783.

In fastchat/serve/gradio_web_server.py, add_text and regenerate have early-return paths that yield a 9-tuple while the normal path returns an 8-tuple. All three callers (textbox.submit, send_btn.click, regenerate_btn.click) wire the output to [state, chatbot, textbox] + btn_list — 8 outputs. The 9-value returns trigger a Gradio output-count mismatch error, so any empty-input submit, conversation-limit hit, or regenerate-without-regen-support crashes the UI.

Root cause

Commit be5db52 (Merge operation & multimodal arena, #3425) removed the image parameter from add_text in the single-model server and dropped the trailing None from the normal return path, but left the same None in the two early-return paths of add_text and the early-return path of regenerate. The extra None was the image slot that no longer exists.

Fix

Drop the leftover None from the three early returns so every path returns the 8 values the output components expect. No other behavior changes.