fix(storage): Fixed a potential null value issue with not checking firstMount.

- Added a check to see if `firstMount` is null to prevent logic errors.
- Adjusted the loading logic of `GetAllRoles` and `GetAllUsers` to only execute when `firstMount` is non-null.
- Fixed the `usedBy` check logic to ensure that an error message is returned under the correct conditions.
- Optimized code structure to reduce unnecessary execution paths.

internal/op/storage.go

@@ -280,6 +280,7 @@ func DeleteStorageById(ctx context.Context, id uint) error {
 		return errors.WithMessage(err, "failed get storage")
 	}
 	firstMount := firstPathSegment(storage.MountPath)
+	if firstMount != "" {
 		roles, err := db.GetAllRoles()
 		if err != nil {
 			return errors.WithMessage(err, "failed to load roles")
@@ -305,6 +306,7 @@ func DeleteStorageById(ctx context.Context, id uint) error {
 		if len(usedBy) > 0 {
 			return errors.Errorf("storage is used by %s, please cancel usage first", strings.Join(usedBy, ", "))
 		}
+	}
 	if !storage.Disabled {
 		storageDriver, err := GetStorageByMountPath(storage.MountPath)
 		if err != nil {