Like @george pointed out, the nick of a user is the text leading up to the exclamation point. To give a little more detail on the structure, turn to the RFC 1459 spec.
The nick is located within the prefix, if one exists. The general structure being:
<server_or_nick>!<user>@<host>
When a prefix is present, the <server_or_nick>is always included, where as both the <user> and <host> are optional and may not be present. If <user> is included it will be prefixed by !, and if <host> is included it will be prefixed with @. In the case of Twitch and ingesting chat messages, I’ve always seen the full prefix included in the message.
On a side note, is the data you posted the exact data that was ingested? Because the prefix, command, and parameters look malformed.