WorkItems rolledup dates: make fixed dates nullable on graphql

What does this MR do and why?

WorkItems rolledup dates: make fixed dates nullable on graphql

When working on the frontend, it was noticed that there was no way to empty the fixed dates (set to null). We decided to change the logic in the backend to be based on the start_date_is_fixed only. In other words, when start_date_is_fixed is true, we assume the frontend is sending the desired fixed dates in start_date_fixed field to be set on the backend. (samething for due_date)

Changelog: added EE: true

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

Screen_Recording_2024-02-12_at_17.03.50

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.