Keep empty locale string

[azatkama]

I have a translation of the phrase
If I delete the locale string of the phrase and run translation:extract command again,
the phrase is filled with id
can you add here https://github.com/schmittjoh/JMSTranslationBundle/blob/master/Model/Message.php#L377
a check for a new message? like this
if ($this->isNew() && !$this->getLocaleString()) { $this->localeString = $message->getLocaleString(); }
thank you