docs: note null/undefined breaking change

closes #10043

Author: dummdidumm

sites/svelte-5-preview/src/routes/docs/content/03-appendix/02-breaking-changes.md

@@ -107,3 +107,7 @@ In Svelte 4, it was possible to specify event attributes on HTML elements as a s
 ```
 
 This is not recommended, and is no longer possible in Svelte 5, where properties like `onclick` replace `on:click` as the mechanism for adding [event handlers](/docs/event-handlers).
+
+### `null` and `undefined` become the empty string
+
+In Svelte 4, `null` and `undefined` were printed as the corresponding string. In 99 out of 100 cases you want this to become the empty string instead, which is also what most other frameworks out there do. Therefore, in Svelte 5, `null` and `undefined` become the empty string.