[BUGFIX] Ensure replacement of assets paths

Replacement of assets paths in concatenated CSS files was only processed for new copies of those assets and skipped for existing ones.
Merge pull request #277 from bjo3rnf/vanishingassetsfix

[BUGFIX] Ensure replacement of assets paths
[BUGFIX] Prevent PHP warning when saving in BE

When saving some content in the BE, the clearCacheCmd hook throws a PHP warning if no vhs-assets cachefiles are present in the typo3temp directory.
Fixed by testing the return of glob() before entering the foreach.
Merge pull request #274 from dogawaf/master

[BUGFIX] Prevent PHP warning when saving in BE