Prevent export crash due to missing user context

[ak4t0sh]

After user data removal by tool_dataprivacy tool_dataprivacy\task\expired_retention_period is used to remove user contexts

When exporting appointments with custom profile field selected this lead to a crash. This is due to profile field visibility check using profile_field_base::is_visible() which is calling context_user::instance() with default strictness (MUST_EXIST)...leading to an exception when the context is removed.