Check existence of data/search_engines/internal.php before Requiring it in update script

Dear Daniel,

As I was trying to update the module today on an instance where it had been installed but never used (I think...) I ran into this error:

php7 PHP Fatal error:  require(): Failed opening required '/var/www/html/corpus-vietnam.fr/modules/AdvancedSearch/data/scripts/../../data/search_engines/internal.php' (include_path='.:/usr/share/php') in /var/www/html/corpus-vietnam.fr/modules/AdvancedSearch/data/scripts/upgrade.php on line 164

This would appear to be simply down to the absence of the file, likely created when the module is actually used I assume? A simple existence check would seem to be in order, which is what allowed me to perform the update.

Thanks for all your hard work on Omeka-S and assorted modules.

Best regards,

Mark.