example_range_test: support empty range

Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>

Author: bajtos

example_range_test.go

@@ -48,8 +48,11 @@ func ExampleDecryptRange() {
 	// Both are known up front, so a handler can write its headers before
 	// decrypting a single chunk.
 	fmt.Printf("Content-Length: %d\n", r.Len())
+	if r.Len() == 0 {
+		fmt.Printf("Content-Range: bytes */%d\n", r.Size())
+		return
+	}
 	fmt.Printf("Content-Range: bytes %d-%d/%d\n", off, off+r.Len()-1, r.Size())
-
 	got, err := io.ReadAll(r)
 	if err != nil {
 		log.Fatal(err)