test: don't drop error on blank IP address in report (#21642)

fixes https://github.com/coder/internal/issues/1286

We can get blank IP address from the net connection if the client has
already disconnected, as was the case in this flake. Fix is to only log
error if we get something non-empty we can't parse.

---------

Co-authored-by: Mathias Fredriksson <mafredri@gmail.com>

agent/agent.go

@@ -882,12 +882,16 @@ const (
 )
 
 func (a *agent) reportConnection(id uuid.UUID, connectionType proto.Connection_Type, ip string) (disconnected func(code int, reason string)) {
-	// Remove the port from the IP because ports are not supported in coderd.
-	if host, _, err := net.SplitHostPort(ip); err != nil {
-		a.logger.Error(a.hardCtx, "split host and port for connection report failed", slog.F("ip", ip), slog.Error(err))
-	} else {
-		// Best effort.
-		ip = host
+	// A blank IP can unfortunately happen if the connection is broken in a data race before we get to introspect it. We
+	// still report it, and the recipient can handle a blank IP.
+	if ip != "" {
+		// Remove the port from the IP because ports are not supported in coderd.
+		if host, _, err := net.SplitHostPort(ip); err != nil {
+			a.logger.Error(a.hardCtx, "split host and port for connection report failed", slog.F("ip", ip), slog.Error(err))
+		} else {
+			// Best effort.
+			ip = host
+		}
 	}
 
 	// If the IP is "localhost" (which it can be in some cases), set it to