Fixes issue #8 by providing a default 'where' value

There's been an issue with PHP7 (or MODX 2.5.7 - not totally sure which) where the user must add a &where=`[]` to the getArchives call to avoid a server 500 error.

This update provides that default if no setting has been passed from the snippet call.

core/components/archivist/elements/snippets/snippet.getarchives.php

@@ -49,7 +49,7 @@
 }
 
 /* get filter by year, month, and/or day. Sanitize to prevent injection. */
-$where = $modx->getOption('where',$scriptProperties,false);
+$where = $modx->getOption('where',$scriptProperties,"[]");
 $where = is_array($where) ? $where : $modx->fromJSON($where);
 $parameters = $modx->request->getParameters();
 
@@ -113,4 +113,4 @@
 } else {
     return 'You must have getResources downloaded and installed to use this snippet.';
 }
-return $output;
+return $output;