Throw exception when site Ids array is empty, to prevent error:
Error: SQLSTATE[42000]: Syntax error or access violation: 1064 You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near ')
                           GROUP BY idsite, date1, date2' at line 5

Payload: /index.php?date=last10&format=rss&idSite=12345%27%22\%27\%22%29;|]*{%0d%0a%3C%00%3E%bf%27%27&method=Actions.getPageUrl&module=API&pageUrl=http://forum.piwik.org/&period=day&token_auth=anonymous&translateColumnNames=1