Bug 1875874 - Allow null distro and distroVersion in legacy telemetry

[BenWu]

This is a follow-up to https://github.com/mozilla-services/mozilla-pipeline-schemas/pull/799 to address https://bugzilla.mozilla.org/show_bug.cgi?id=1875874 and https://bugzilla.mozilla.org/show_bug.cgi?id=1877339.

Related incident doc: https://docs.google.com/document/d/1M7ageyDdS8sha0vYTbWwJrsRylrJgUYArjogCpHeL2Q/edit

This change allows distro and distroVersion to be present and null instead of either not present or present and non-null.

Checklist for reviewer:

• [ ] Commits should reference a bug or github issue, if relevant (if a bug is referenced, the pull request should include the bug number in the title)
• [ ] If adding a new field, the field should have a description (see #576 for an example)
• [ ] If coming from a fork, run integration tests: ./.github/push-to-trigger-integration <username>:<branchname>

For glean changes:

• [ ] Update templates/include/glean/CHANGELOG.md

For modifications to schemas in restricted namespaces (see CODEOWNERS):

• [ ] Follow the change control procedure