Fix panic when failing to load RRD size

[emilk]

What

• Introduced in Show download sizes of in the example page #4841

poll-promise has a gotcha: if you drop the Sender without sending a result,, the Promise will panic when polled.

I opened an issue about this: EmbarkStudios/poll-promise#25

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 the web demo (if applicable):
  • Using newly built examples: app.rerun.io
  • Using examples from latest main build: app.rerun.io
  • Using full set of examples from nightly build: app.rerun.io
• The PR title and labels are set such as to maximize their usefulness for the next release's CHANGELOG

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[teh-cmc]

Suggested change

	sender.send(None);
	// TODO(EmbarkStudios/poll-promise#25): if you drop the `Sender` without sending
	// a result, the `Promise` will panic when polled.
	// See <https://github.com/EmbarkStudios/poll-promise/issues/25>.
	sender.send(None);

[emilk]

This is a not a TODO - we should always send something, otherwise the promise has nothing to resolve to.
That is: this PR fixes a real bug; the linked poll-promise issue is about what that bug should cause.