Fill missing dates on VSA duration chart

What does this MR do and why?

This MR invokes Gitlab::Analytics::DateFiller utility function when generating data for the duration chart in VSA. Reason: we want to return data (even if it's null) for every date within the requested range.

Screenshot: !97230 (comment 1096608446)

How to set up and validate locally

1. Ensure that you're on Ultimate. Seed the VSA feature:

   SEED_CYCLE_ANALYTICS=true  SEED_VSA=true FILTER=cycle_analytics rake db:seed_fu

2. Navigate to the Group.
3. Go to Analytics > Value Stream Analytics and create a value stream.
4. Aggregate the data in rails console

   Analytics::CycleAnalytics::IncrementalWorker.new.perform

5. Go to the generated group and create a few group labels and assign them to issues
6. Go to Analytics > Value Stream Analytics, data should show up. On the bottom, you should see your labels.
7. See if the chart renders

MR acceptance checklist

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