* Use "filesystem_cache_readonly" config setting, update comment in config.sample
* Use $this->cacheActive to cache config setting
* Add public Scanner::setCacheActive() to set $cacheActive programmatically

so that descendant classes can access them.

* test case added to avoid adding them later

Added extra code to handle the case of Oracle which saves empty strings
as null values.

This patch was automatically generated as part of the following inspection:
https://scrutinizer-ci.com/g/owncloud/core/inspections/cdfecc4e-a37e-4233-8025-f0d7252a8720

Enabled analysis tools:
 - PHP Analyzer
 - JSHint
 - PHP Copy/Paste Detector
 - PHP PDepend

The encrypted size was used when calculating folder sizes.
This fix now also sums up the unencrypted size and shows that one when
available.

Fixes #6526