Fix TypeError from HEALPix grid.node_lat when not yet populated

[Sevans711]

Closes #1637

Overview

Grid.node_lat was failing on any HEALPix grid when node_lat was not already been populated. For example, ux.Grid.from_healpix(zoom=2).node_lat was causing TypeError.

The fix here is exactly the one proposed in that issue:

Fix is _populate_healpix_boundaries(self._ds) at grid.py:1003.

Adds a regression test which would have failed before but now succeeds.

PR Checklist

General

• An issue is created and linked
• Added appropriate labels (if your uxarray repo permissions allow it)
• Filled out Overview and Expected Usage (if applicable) sections

Testing & Benchmarking

• [N/A] Adequate tests are created if there is new functionality
• Tests are not too basic (such as simply calling a function and nothing else)
• [N/A] Tests cover all major paths in your new functions
• [N/A] If this PR could affect performance, ran ASV benchmarks and confirmed they show expected behavior (add a new benchmark if necessary)

AI Disclosure

AI Usage: just GitHub Copilot inline code suggestions.

• I take responsibility for all AI-generated content in my PR.
• I have tested all AI-generated content in my PR.