Fix empty user/group detection on chuser

This should fix #1216.

cmd/yggdrasil/chuser_unix.go

@@ -14,6 +14,12 @@ import (
 
 func chuser(input string) error {
 	givenUser, givenGroup, _ := strings.Cut(input, ":")
+	if givenUser == "" {
+		return fmt.Errorf("user is empty")
+	}
+	if strings.Index(input, ":") > -1 && givenGroup == "" {
+		return fmt.Errorf("group is empty")
+	}
 
 	var (
 		err      error

cmd/yggdrasil/chuser_unix_test.go

@@ -4,8 +4,8 @@
 package main
 
 import (
-	"testing"
 	"os/user"
+	"testing"
 )
 
 // Usernames must not contain a number sign.