Codegen a nullptr check when forwarding from component to datatype (#…

…7430)

### What
We already have a similar check in our builders, but when a component
forwards to the datatype builder, we need a similar check.

### 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/7430?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/7430?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/7430)
- [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/build/re_types_builder/src/codegen/cpp/mod.rs

@@ -1505,7 +1505,13 @@ fn to_arrow_method(
         (
             true,
             quote! {
-                return Loggable<#forwarded_type>::to_arrow(&instances->#field_name, num_instances);
+                if (num_instances == 0) {
+                    return Loggable<#forwarded_type>::to_arrow(nullptr, 0);
+                } else if (instances == nullptr) {
+                    return rerun::Error(ErrorCode::UnexpectedNullArgument, "Passed array instances is null when num_elements > 0.");
+                } else {
+                    return Loggable<#forwarded_type>::to_arrow(&instances->#field_name, num_instances);
+                }
             },
         )
     } else {