Bump core from ac93dfa to 913aa03 (#994)

Bumps [core](https://github.com/microsoft/typespec) from `ac93dfa` to
`913aa03`.
<details>
<summary>Commits</summary>
<ul>
<li><a
href="https://github.com/microsoft/typespec/commit/913aa03f78cf5b85b03c5efc5a8666b45f48d514"><code>913aa03</code></a>
fix the nullable value type (<a
href="https://redirect.github.com/microsoft/typespec/issues/3567">#3567</a>)</li>
<li><a
href="https://github.com/microsoft/typespec/commit/1f82d65ea04134d7182158cf3cc98158b6923f87"><code>1f82d65</code></a>
Do not run decorators on cloned type if the original type wasn't
finished (<a
href="https://redirect.github.com/microsoft/typespec/issues/3">#3</a>...</li>
<li>See full diff in <a
href="https://github.com/microsoft/typespec/compare/ac93dfa318bb02217b99d0aa3b0898c725d88c0a...913aa03f78cf5b85b03c5efc5a8666b45f48d514">compare
view</a></li>
</ul>
</details>
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