Fix: Handle nullable collection getters in svelte-db useLiveQuery

[elohmeier]

🎯 Changes

When useLiveQuery receives a getter function that returns null during SSR (e.g., () => documentsCollection where documentsCollection is initially null), the implementation was trying to spread null into a config object, causing an error.

This adds a null/undefined check before spreading to return null early, matching how react-db and solid-db handle this case.

Before: useLiveQuery(() => nullableCollection) throws when getter returns null
After: Returns empty/idle state, then updates reactively when collection becomes available

✅ Checklist

• I have followed the steps in the Contributing guide.
• I have tested this code locally with pnpm test:pr.

🚀 Release Impact

• This change affects published code, and I have generated a changeset.
• This change is docs/CI/dev-only (no release).

[changeset-bot]

🦋 Changeset detected

Latest commit: 33d87a3

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@tanstack/svelte-db	Patch

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commit: 33d87a3