[GH-168] DebugStack should ignore stopQuery when disabled

This issue is created automatically through a Github pull request on behalf of dpb587:

Url: https://github.com/doctrine/dbal/pull/168

Message:

I noticed that DebugStack was adding an empty element when it was disabled. This PR changes it to match startQuery with its behavior.

1) Doctrine\Tests\DBAL\Logging\DebugStackTest::testLoggedQueryDisabled
Failed asserting that two arrays are equal.
— Expected
+++ Actual
@@ @@
Array
(
+ [0] => Array
+ (
+ [executionMS] => 1341871660.9651
+ )
+
)

I've submitted the PR to 2.0.x since that's where the bug is first seen and is still listed as stable on doctrine-project.org. Preferably this could also be applied to newer branches as well, or I can also submit separate PRs.