fix: Fix unmodified placeholder replacing the actual value when updating

fix: Fix unmodified placeholder replacing the actual value when updating

When updating global storages and user storages a property is not
updated by "StoragesService::updateStorage()" if the value matches the
unmodified placeholder. However, userglobal storages are not updated
through the "StoragesService"; as only the authentication mechanism is
updated it is directly done with "saveBackendOptions()" in
"IUserProvided" or "UserGlobalAuth". Due to this the unmodified
placeholder value needs to be explicitly checked in those cases and
replaced by the actual value (note that in this case it is not possible
to just skip updating a specific property).

Signed-off-by: Daniel Calviño Sánchez <danxuliu@gmail.com>