Fixes to NaN padding in HiPS

[astrofrog]

In some cases the padding had zero values

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.06%. Comparing base (939393b) to head (436a361).
⚠️ Report is 6 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #575   +/-   ##
=======================================
  Coverage   91.06%   91.06%           
=======================================
  Files          30       30           
  Lines        1858     1858           
=======================================
  Hits         1692     1692           
  Misses        166      166           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.