net/http: add a test for an empty ServeMux

Make sure a ServeMux with no patterns is well-behaved.

Updates #61410.

Change-Id: Ib3eb85b384e1309e785663902d2c45ae01e64807
Reviewed-on: https://go-review.googlesource.com/c/go/+/530479
Reviewed-by: Damien Neil <dneil@google.com>
Run-TryBot: Jonathan Amsterdam <jba@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

src/net/http/server.go

@@ -33,8 +33,6 @@ import (
 	"golang.org/x/net/http/httpguts"
 )
 
-// TODO(jba): test
-
 // Errors used by the HTTP server.
 var (
 	// ErrBodyNotAllowed is returned by ResponseWriter.Write calls

src/net/http/server_test.go

@@ -118,6 +118,20 @@ func TestFindHandler(t *testing.T) {
 	}
 }
 
+func TestEmptyServeMux(t *testing.T) {
+	// Verify that a ServeMux with nothing registered
+	// doesn't panic.
+	mux := NewServeMux()
+	var r Request
+	r.Method = "GET"
+	r.Host = "example.com"
+	r.URL = &url.URL{Path: "/"}
+	_, p := mux.Handler(&r)
+	if p != "" {
+		t.Errorf(`got %q, want ""`, p)
+	}
+}
+
 func TestRegisterErr(t *testing.T) {
 	mux := NewServeMux()
 	h := &handler{}