Fix #13638: Svelte 5: Custom element's default <slot> not created when n

[danielalanbates]

Fixes #13638

Summary

This PR fixes: Svelte 5: Custom element's default not created when no initial children are passed

Changes

.../internal/client/dom/elements/custom-element.js | 14 ++++++------
 .../no-initial-children-default-slot/_config.js    | 25 ++++++++++++++++++++++
 .../no-initial-children-default-slot/main.svelte   |  3 +++
 3 files changed, 35 insertions(+), 7 deletions(-)

Testing

Please review the changes carefully. The fix was verified against the existing test suite.

---

This PR was created with the assistance of Claude Sonnet 4.6 by Anthropic | effort: low. Happy to make any adjustments!

By submitting this pull request, I confirm that my contribution is made under the terms of the project's license (contributor license agreement).

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