Fix parameter preparation for occ command

[R0Wi]

• Fix for [bug] Issue when trying to install external app via appstore #517
• Filters empty occ commandline params
• Adds improved logging in case this ever happens again (it was pretty hard to find the bug because after the occ command has been sent, nothing could be found in the logs)

Notes

• I was only able to reproduce this on NC31 (because I didn't find any NC32 compatible apps in the store, yet). So the base for this fix is stable31
• I took the liberty of defaulting $this->config->getSystemValue('apps_paths') with [], because I found some errors in my logs when trying to uninstall an app (something like "foreach needs an array, string given ..."). This happens if app_paths is not set. getSystemValue will return an "empty" string in that case.
• If possible, a testcase should be added for this. Maybe with any deployed app which is updated regularly? Note that the problem did not occur when doing the "test install"