Fallback to system time zone on calendar creation with an invalid one.

Summary:
ensure to always have a valid time zone defined
for a calendar. Fallback to the system time zone if
provided time zone object or ID is invalid.

Test Plan:
add a test in memory calendar to verify
that creating a calendar with an invalid time zone
results in the system one being used.

Reviewers: KDE PIM, vkrause

Reviewed By: KDE PIM, vkrause

Subscribers: kde-pim

Tags: KDE PIM

Differential Revision: https://phabricator.kde.org/D25367