fix: don't crash on undefined `document.contentType` (#17707)

closes #17706

.changeset/curvy-pants-hide.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: don't crash on undefined `document.contentType`

packages/svelte/src/internal/client/constants.js

@@ -67,7 +67,7 @@ export const STALE_REACTION = new (class StaleReactionError extends Error {
 	message = 'The reaction that called `getAbortSignal()` was re-run or destroyed';
 })();
 
-export const IS_XHTML = /* @__PURE__ */ globalThis.document?.contentType.includes('xml') ?? false;
+export const IS_XHTML = /* @__PURE__ */ globalThis.document?.contentType?.includes('xml') ?? false;
 export const ELEMENT_NODE = 1;
 export const TEXT_NODE = 3;
 export const COMMENT_NODE = 8;