Fix missing default blacklist.

The default blacklist was no longer being used, because the line
$wgSpamBlacklistFiles =& $wgBlacklistSettings['spam']['files']
initialized $wgBlacklistSettings['spam']['files'] to null,
and BaseBlacklist::__construct then overrode $this->files
with null.

Change-Id: I22448bfb87eef6dd86b61362f3eb6bb2198a10b6