Correctly display null timestamps in the dataframe view (rerun-io#7659)

### What

The dataframe view was previously ignoring the validity of data in time
columns. This PR fixes this and properly displays `null`.

<img width="1307" alt="image"
src="https://github.com/user-attachments/assets/526c1fa6-a4e4-46a1-a56b-24a54ae1f08f">


### 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 examples from latest `main` build:
[rerun.io/viewer](https://rerun.io/viewer/pr/7659?manifest_url=https://app.rerun.io/version/main/examples_manifest.json)
* Using full set of examples from `nightly` build:
[rerun.io/viewer](https://rerun.io/viewer/pr/7659?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
* [x] If applicable, add a new check to the [release
checklist](https://github.com/rerun-io/rerun/blob/main/tests/python/release_checklist)!
* [x] If have noted any breaking changes to the log API in
`CHANGELOG.md` and the migration guide

- [PR Build Summary](https://build.rerun.io/pr/7659)
- [Recent benchmark results](https://build.rerun.io/graphs/crates.html)
- [Wasm size tracking](https://build.rerun.io/graphs/sizes.html)

To run all checks from `main`, comment on the PR with `@rerun-bot
full-check`.

crates/viewer/re_space_view_dataframe/src/display_record_batch.rs

@@ -247,14 +247,18 @@ impl DisplayColumn {
                     return;
                 }
 
-                let timestamp = TimeInt::try_from(time_data.value(row_index));
-                match timestamp {
-                    Ok(timestamp) => {
-                        ui.label(timeline.typ().format(timestamp, ctx.app_options.time_zone));
-                    }
-                    Err(err) => {
-                        ui.error_label(&format!("{err}"));
+                if time_data.is_valid(row_index) {
+                    let timestamp = TimeInt::try_from(time_data.value(row_index));
+                    match timestamp {
+                        Ok(timestamp) => {
+                            ui.label(timeline.typ().format(timestamp, ctx.app_options.time_zone));
+                        }
+                        Err(err) => {
+                            ui.error_label(&format!("{err}"));
+                        }
                     }
+                } else {
+                    ui.label("-");
                 }
             }
             Self::Component {