Fix panic when failing to load RRD size (#4872)

### What
* Introduced in #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
* [x] I have read and agree to [Contributor
Guide](https://github.com/rerun-io/rerun/blob/main/CONTRIBUTING.md) and
the [Code of
Conduct](https://github.com/rerun-io/rerun/blob/main/CODE_OF_CONDUCT.md)
* [x] I've included a screenshot or gif (if applicable)
* [x] I have tested the web demo (if applicable):
* Using newly built examples:
[app.rerun.io](https://app.rerun.io/pr/4872/index.html)
* Using examples from latest `main` build:
[app.rerun.io](https://app.rerun.io/pr/4872/index.html?manifest_url=https://app.rerun.io/version/main/examples_manifest.json)
* Using full set of examples from `nightly` build:
[app.rerun.io](https://app.rerun.io/pr/4872/index.html?manifest_url=https://app.rerun.io/version/nightly/examples_manifest.json)
* [x] The PR title and labels are set such as to maximize their
usefulness for the next release's CHANGELOG

- [PR Build Summary](https://build.rerun.io/pr/4872)
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crates/re_viewer/src/ui/welcome_screen/example_page.rs

@@ -158,6 +158,7 @@ fn load_file_size(egui_ctx: &egui::Context, url: String) -> Promise<Option<u64>>
                         response.status,
                         response.status_text
                     );
+                    sender.send(None);
                 }
             }
             Err(err) => {