Fix projectAnalytics.aggregate.durationStatistics nil values

What does this MR do and why?

This MR fixes projectAnalytics.aggregate.durationStatistics so that it returns 0 instead of nil values for the requested percentiles if there are no finished pipelines for the period.

Changelog: fixed

References

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• GraphQL: `projectAnalytics.aggregate.durationSt... (#514378 - closed)

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Before	After

How to set up and validate locally

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

1. Run the following query in http://gdk.test:3000/-/graphql-explorer:

   {
     project(
       fullPath: "gitlab-org/gitlab-shell"
     ) {
       pipelineAnalytics {
         aggregate {
           count
           successCount: count(status: SUCCESS)
           failedCount: count(status: FAILED)
           durationStatistics {
             p50
           }
         }
       }
     }
   }

In this branch, p50 should be zero if there are no pipelines executed for that project (which is likely to be the case). In master it returns nil.