Fix start_is_fixed and due_date_is_fixed default

What does this MR do and why?

Fix start_is_fixed and due_date_is_fixed default

When creating a work item where the start_date or due_date param was set but nil, we still set start_date_is_fixed and due_date_is_fixed as true, even when the parameter was set to false.

This caused an issue when syncing epic work items, where we created dates_source with the wrong default values.

Issue: #446178 (closed)

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