… and old set up with no arrays (#239)

## Description
- PR to fix #238

Add test to write metadata with no arrays, and when arrays dict contains
multiple entries (simulating multiple-dataset set up)

## What problem does this change solve?
Previous changes broke support for old set ups where supporting arrays
were not defined.

## What issue or task does this change relate to?
[<!-- link to Issue Number
-->](#238)

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## Description
Further improve typing on registry

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<details>
<summary>Changed files</summary>
<ul>
<li>synced local directory <code>./</code> with remote directory
<code>sync-files/anemoi/all/</code></li>
</ul>
</details>

---

This PR was created automatically by the
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## Description
<!-- What issue or task does this change relate to? -->

## What problem does this change solve?
<!-- Describe if it's a bugfix, new feature, doc update, or breaking
change -->

## What issue or task does this change relate to?
<!-- link to Issue Number -->

##  Additional notes ##
<!-- Include any additional information, caveats, or considerations that
the reviewer should be aware of. -->

***As a contributor to the Anemoi framework, please ensure that your
changes include unit tests, updates to any affected dependencies and
documentation, and have been tested in a parallel setting (i.e., with
multiple GPUs). As a reviewer, you are also responsible for verifying
these aspects and requesting changes if they are not adequately
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🤖 Automated Release PR

This PR was created by `release-please` to prepare the next release.
Once merged:

1. A new version tag will be created
2. A GitHub release will be published
3. The changelog will be updated

Changes to be included in the next release:
---


##
[0.4.41](0.4.40...0.4.41)
(2026-01-08)


### Features

* Fix argument type for transfer tool
([#253](#253))
([7d020d1](7d020d1))


### Bug Fixes

* Further improve typing on registry
([#249](#249))
([97c8874](97c8874))
* Save_metadata to support both multiple datasets with many arrays and
old set up with no arrays
([#239](#239))
([b22444f](b22444f))

---
> [!IMPORTANT]
> Please do not change the PR title, manifest file, or any other
automatically generated content in this PR unless you understand the
implications. Changes here can break the release process.
> ⚠️ Merging this PR will:
> - Create a new release
> - Trigger deployment pipelines
> - Update package versions

 **Before merging:**
 - Ensure all tests pass
 - Review the changelog carefully
 - Get required approvals

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