using array_key_exists() instead of isset() - required because in case the value is null isset is returning false

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

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Issue #7111

On SQLite the app order can be arbitrary and cause strange bugs.
On MySQL, the app order seems to be always alphabetical.

This fix enforces alphabetical order to make sure that all environments
behave the same and to reduce bugs related to app loading order.

Fixes #6442

Conflicts:
	lib/private/vcategories.php

Implemented unittest for OC\AppConfig

This is NoSpaceAfterComma

Fixed a bug in appconfig, where an initially empty set value led to endless duplicates of this app/key pair.

Path by Elias Probst <mail@eliasprobst.eu>

Refactoring OC_APPCONFIG and OC_PREFERENCES, minor changes to OC_APP. These libraries are skeletons only, the functions need to be written.