fix(types): Allow Snippet mapper with primitives to match undefined

[xeho91]

Resolves #214

📦 Published PR as canary version: 4.1.8--canary.215.f10113b.0

✨ Test out this PR locally via:

npm install @storybook/[email protected]
# or 
yarn add @storybook/[email protected]

[xeho91]

There's a breakdown of existing svelte-check issues.

• https://github.com/storybookjs/addon-svelte-csf/actions/runs/11379831712/job/31657979250?pr=215#step:7:23
• https://github.com/storybookjs/addon-svelte-csf/actions/runs/11379831712/job/31657979250?pr=215#step:7:31
• https://github.com/storybookjs/addon-svelte-csf/actions/runs/11379831712/job/31657979250?pr=215#step:7:37

Those are related to @storybook/svelte types. Those stories tests are using regular CSF.

---

• https://github.com/storybookjs/addon-svelte-csf/actions/runs/11379831712/job/31657979250?pr=215#step:7:57
• https://github.com/storybookjs/addon-svelte-csf/actions/runs/11379831712/job/31657979250?pr=215#step:7:65
• https://github.com/storybookjs/addon-svelte-csf/actions/runs/11379831712/job/31657979250?pr=215#step:7:72
• https://github.com/storybookjs/addon-svelte-csf/actions/runs/11379831712/job/31657979250?pr=215#step:7:85
• https://github.com/storybookjs/addon-svelte-csf/actions/runs/11379831712/job/31657979250?pr=215#step:7:92

Those are caused by legacy components of this addon, and they are using Svelte 4 features.
At this moment, I have no clue how to fix it.

[xeho91]

Superseded by #219