fix folders validation

Fixes #510

dataprovider/dataprovider.go

@@ -1534,6 +1534,7 @@ func createUserPasswordHash(user *User) error {
 // ValidateFolder returns an error if the folder is not valid
 // FIXME: this should be defined as Folder struct method
 func ValidateFolder(folder *vfs.BaseVirtualFolder) error {
+	folder.FsConfig.SetEmptySecretsIfNil()
 	if folder.Name == "" {
 		return util.NewValidationError("folder name is mandatory")
 	}