all: use cmpx.Or where it made sense

I left a few out where writing it explicitly was better
for various reasons.

Updates #8296

Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

tsweb/tsweb.go

@@ -27,6 +27,7 @@ import (
 	"tailscale.com/net/tsaddr"
 	"tailscale.com/tsweb/varz"
 	"tailscale.com/types/logger"
+	"tailscale.com/util/cmpx"
 	"tailscale.com/util/vizerror"
 )
 
@@ -144,10 +145,7 @@ func (h Port80Handler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 		// Redirect authorized user to the debug handler.
 		path = "/debug/"
 	}
-	host := h.FQDN
-	if host == "" {
-		host = r.Host
-	}
+	host := cmpx.Or(h.FQDN, r.Host)
 	target := "https://" + host + path
 	http.Redirect(w, r, target, http.StatusFound)
 }