ViewProperties: Return nullptr if viewPropertiesString is empty

ViewProperties: Return nullptr if viewPropertiesString is empty

If viewPropertiesString is empty, return a nullptr.
This will later used in the stack by the defaultProperties call.

In defaultProperties, if we can't find the global directory,
create new one with a tempfile. If tempfiles can't be created,
use default instead.

This will ensure that view settings are saved and loaded correctly
if user has separate view properties per folder.

This will also add an unit test, where we create a global directory,
modify it and make sure the changes are reflected in the unmodified
folder.

BUG:495878