Special case 0 uploads in upgrade.

I don't know that this is the proper issue to put this in, but it was due to an update from the 4.7.x version to the 5.x version so I'm going to add here.

I got the following error updating to the 5.x version:

    * CREATE TABLE {comment_upload_files} ( `fid` int(10) unsigned NOT NULL default '0', `nid` int(10) unsigned NOT NULL default '0', `cid` int NOT NULL default '0', `filename` varchar(255) NOT NULL default '', `filepath` varchar(255) NOT NULL default '', `filemime` varchar(255) NOT NULL default '', `filesize` int(10) unsigned NOT NULL default '0', description varchar(255) NOT NULL default '', list tinyint(1) unsigned NOT NULL default 0, PRIMARY KEY (`fid`) ) /*!40100 DEFAULT CHARACTER SET utf8 */;
    * Failed: INSERT INTO {sequences} (name, id) VALUES('comment_upload_files_fid', )
    * DROP TABLE {comment_files}

The install fails to include the value for the sequence id. I didn't actually have any uploaded files attached to comments, which could explain this. I don't know if this will actually cause problems - I'm just playing with it on my 5.x install on my laptop - but I thought I should come down here and find out. At the least the error should be made to not occur so that it doesn't make people like me worry :)