Fixed a chat failure on agents with no instructions or site branding

An agent using the Custom engine with no instructions configured, on a site with no vertical, site information, or tone of voice set, could fail to respond partway through a shopper's turn in the public chat.

The underlying system prompt sent to the model came out completely empty in that specific setup, and it was rejected outright instead of being treated as "no system prompt." It's fixed now: an agent in this configuration answers normally. No action needed on your side, but if you haven't yet, adding instructions or site branding still shapes how your agent responds.