Remove incorrect empty nick/ident/hostname.

Per RFC 1459:

<prefix>   ::= <servername> | <nick> [ '!' <user> ] [ '@' <host> ]
<nick>     ::= <letter> { <letter> | <number> | <special> }
<user>     ::= <nonwhite> { <nonwhite> }
<host>     ::= see RFC 952 [DNS:4] for details on allowed hostnames

and per RFC 952:

<official hostname> ::= <hname>
<hname> ::= <name>*[.<name>]
<name>  ::= <let>[*[<let-or-digit-or-hyphen>]<let-or-digit>]

Author: progval

tests/userhost-split.yaml

@@ -34,26 +34,11 @@ tests:
       host: "localhost"
 
   # without atoms
-  - source: "[email protected]"
-    atoms:
-      user: "ag"
-      host: "127.0.0.1"
-
-  - source: "[email protected]"
-    atoms:
-      nick: "coolguy"
-      host: "127.0.0.1"
-
   - source: "[email protected]"
     atoms:
       nick: "coolguy"
       host: "127.0.0.1"
 
-  - source: "coolguy!ag@"
-    atoms:
-      nick: "coolguy"
-      user: "ag"
-
   - source: "coolguy!ag"
     atoms:
       nick: "coolguy"