Reset default value for attrs in IndexSeries

[rly]

Summary of changes

• Fix Suggestion: Use "default_value: null" for unused fields in IndexSeries #641

Checklist

For all schema changes:

• Add release notes for the PR to docs/format/source/format_release_notes.rst.
• Have you included the relevant issue number using "Fix #XXX" notation where XXX is the issue number? By including "Fix #XXX" you allow GitHub to close issue #XXX when the PR is merged.
• Make sure that hdmf-common-schema points to the latest release and not the latest commit on the main branch.

[rly]

This is proving to be tricky to get right in PyNWB, so I think we should punt this change to the next release.