errrr this seems to work?

packages/svelte/src/compiler/phases/3-transform/client/transform-client.js

@@ -444,6 +444,7 @@ export function client_component(source, analysis, options) {
 		}
 
 		const hmr = b.block([
+			b.stmt(b.assignment('??=', b.id('import.meta.hot.data'), b.object([]))),
 			b.stmt(
 				b.assignment(
 					'??=',

packages/svelte/tests/snapshot/samples/hmr/_expected/client/index.svelte.js

@@ -10,6 +10,7 @@ function Hmr($$anchor) {
 }
 
 if (import.meta.hot) {
+	import.meta.hot.data ??= {};
 	import.meta.hot.data.source ??= $.source(Hmr);
 
 	const $$filename = Hmr[$.FILENAME];