Fix for partial regression in r84534: Block::newFromId was no longer handling the...

Fix for partial regression in r84534: Block::newFromId was no longer handling the case where there was no matching row correctly. Now returns null.

This bug was showing up in test results like this:

  BlockTest::testInitializerFunctionsReturnCorrectBlock
  Trying to get property of non-object

  /home/ci/cruisecontrol-bin-2.8.3/projects/mw/source/includes/Block.php:340
  /home/ci/cruisecontrol-bin-2.8.3/projects/mw/source/includes/Block.php:365
  /home/ci/cruisecontrol-bin-2.8.3/projects/mw/source/includes/Block.php:118
  /home/ci/cruisecontrol-bin-2.8.3/projects/mw/source/tests/phpunit/includes/BlockTest.php:60
  /home/ci/cruisecontrol-bin-2.8.3/projects/mw/source/tests/phpunit/MediaWikiTestCase.php:60
  /home/ci/cruisecontrol-bin-2.8.3/projects/mw/source/tests/phpunit/MediaWikiPHPUnitCommand.php:20
  /home/ci/cruisecontrol-bin-2.8.3/projects/mw/source/tests/phpunit/phpunit.php:60

However, that only triggers because the actual test is failing -- it's expecting to get a return back. This can only be reproduce when using the suite.xml configuration file, and not when running the BlockTest standalone.