Emit null pushed_at instead of a zero date for commits

[Tarasusrus]

What this PR does

Fixes #469.

The Commits (and Commit Files) query types show an incorrect pushed_at timestamp — every row is 0001-01-01.

The root cause is that GitHub deprecated the GraphQL Commit.pushedDate field and now always returns null for it. Verified against the live API:

{ repository(name:"github-datasource", owner:"grafana") {
    object(expression:"main") { ... on Commit { history(first:5) { nodes {
      committedDate authoredDate pushedDate
} } } } } }

→ pushedDate is null for every commit (committedDate/authoredDate are populated).

The code mapped that null straight into a non-nullable time.Time field, so it surfaced as the Go zero date 0001-01-01 on every row.

Fix

Make pushed_at nullable (*time.Time) and emit nil when the pushed date is absent, so it renders as a null cell instead of a bogus zero date. Applied to both the commits and commit-files frames via a small nullableTime helper.

Tests

Updated TestCommitsDataframe so the first commit has no pushed date (matching real API behavior) and the second keeps one, exercising both the null and populated paths. Golden files regenerated — pushed_at is now "nullable": true and the null commit yields null. go test ./pkg/... is green.

[cla-assistant]

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[cla-assistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}

[cla-assistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}

[mattvella07]

LGTM