Fix session warning in PHP 7.1
Fix session warning in PHP 7.1

ElanMan/simple-quiz#31

This patch fixes the warning on line 22 of Session.php under PHP 7.1 (tested in 7.1.11 under AMI Linux). Note that I did not experience the session_regenerate_id() issue in the referenced issue.

NB: SessionHandlerInterface requires the [read method](http://php.net/manual/en/sessionhandlerinterface.read.php) return a string for all values, else a PHP warning is issued. This commit allows the function to return an empty string, whereas it was previously not returning a value.
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