We were sending an empty string to the output channel when `ping`
messages were received from Anthropic's API. This was causing the TUI
to break since we were doing an empty chunk check (and mistakenly not
waiting for future chunks if one was received).
This commit makes it so we no longer an empty string on the ping
message from Anthropic, and, we update the handling of
msgAssistantChunk and msgAssistantReply to make it less likely that we
forget to wait for the next chunk/reply.
