Fix bug when joining cleared optional components

[jleibs]

What

Resolves the second issue identified from:

• log_line_strips_3d with no stroke_widths argument shows no lines #3711

Even with the fix from:

• Fix default stroke width handling in log_line_strip_Xd and log_obbs #3720

We could still reproduce an error with code such as:

"""Log a simple line strip."""
import rerun as rr

rr.init("rerun_example_line_strip3d", spawn=True)

points = [
    [0, 0, 0],
    [0, 0, 1],
    [1, 0, 0],
    [1, 0, 1],
    [1, 1, 0],
    [1, 1, 1],
    [0, 1, 0],
    [0, 1, 1],
]

rr.log("strip", rr.LineStrips3D([points], radii=[]))

Which would still display no lines.

It turns out the single-row joining iterator introduced a bug when a component is cleared. This yields an empty cell for the component, but still passes the key-based check that was being done, and then subsequently yields no items.

This PR adds a unit-test reproing the issue, a debug_assertion where we were previously violating an assumption, and now guards against the condition.

Checklist

• I have read and agree to Contributor Guide and the Code of Conduct
• I've included a screenshot or gif (if applicable)
• I have tested demo.rerun.io (if applicable)

• PR Build Summary
• Docs preview
• Examples preview
• Recent benchmark results
• Wasm size tracking

[emilk]

Good catch, but I'm nervous we still have the bug for splats!

[emilk]

Please motivate this with a comment, i.e. why only non-empty components?

And does the code handle splat components (length=1) correctly?

[emilk]

again, I don't think this is true for splats, e.g. Points3D(positions=[a,b,c,d], colors=[rgba])

[jleibs]

Splats are currently split into their own rows. See: #1893 -- adding a TODO linking to that issue though.

[jleibs]

Good call on splats. We technically don't have a problem (yet) because splats are currently logged to a separate row, but I will add a unit test to make sure we don't regress on that front when we change that assumption.