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 started 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.
