Make Block::newFromTarget() work again when passing only a vague target

Make Block::newFromTarget() work again when passing only a vague target

Since SVN r106354 (85ee2d2d), when passing null or an invalid block target
to Block::newFromTarget(), it was never returning any block, even if the
vague target would have matched one.

This broke two features of core MediaWiki:
- Excluding user and user talk pages of blocked users from being indexed,
  the feature that actually caused bug 33101 and the revision mentionned above
- Blocking of account creation when both the user and its IP address are blocked,
  but of only the IP address blocks prevents account creation (bug 13611)
And maybe some others in extensions, I didn't check that.

This changes reverts part of r106354 to make Block::newFromTarget() work again
in that case and changed Article::getRobotPolicy() to pass the user to be checked
as vague target only when it's an IP address and as specific target otherwise.

Change-Id: Ie7e16e0bae8c4326d16cca237877693f7b474a01