Fix attempt of traversing bool in FileLockRegion

Minor fix: on some platforms (i.e. ArchLinux) `glob()` returns `false` when nothing matched even though no errors occurred. As docs for `glob` say, such behavior is expected (see ‘Note’ at http://php.net/glob)

This made 1 existing test fail:

```
./vendor/bin/phpunit 
PHPUnit 4.3-g58f3a0e by Sebastian Bergmann.

Configuration read from /home/alex/git/doctrine2/phpunit.xml.dist

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Time: 14.37 seconds, Memory: 164.50Mb

There was 1 error:

1) Doctrine\Tests\ORM\Cache\FileLockRegionTest::testEvictAll
Invalid argument supplied for foreach()

/home/alex/git/doctrine2/lib/Doctrine/ORM/Cache/Region/FileLockRegion.php:204
/home/alex/git/doctrine2/tests/Doctrine/Tests/ORM/Cache/AbstractRegionTest.php:80
```

Traversing bool is no good and casting to array wouldn't hurt there as far as I can tell.