add file validation to print_nbt command

cmd/strom/print_nbt/main.go

@@ -4,6 +4,7 @@ import (
 	"bytes"
 	"encoding/json"
 	"flag"
+	"fmt"
 	"os"
 
 	"github.com/admin-else/strom/nbt"
@@ -20,6 +21,9 @@ func Run(args []string) (err error) {
 	if err != nil {
 		return
 	}
+	if *FileFlag == "" {
+		return fmt.Errorf("no file specified")
+	}
 	f, err := os.Open(*FileFlag)
 	if err != nil {
 		return