fix: correctly handle falsy values of style directives in SSR mode (#11584)

fixes #11044

.changeset/long-owls-eat.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: correctly handle falsy values of style directives in SSR mode

packages/svelte/src/runtime/internal/ssr.js

@@ -193,7 +193,7 @@ export function add_classes(classes) {
 /** @returns {string} */
 function style_object_to_string(style_object) {
 	return Object.keys(style_object)
-		.filter((key) => style_object[key])
+		.filter((key) => style_object[key] != null && style_object[key] !== '')
 		.map((key) => `${key}: ${escape_attribute_value(style_object[key])};`)
 		.join(' ');
 }

packages/svelte/test/runtime/samples/inline-style-directive-falsy-value/_config.js

@@ -0,0 +1,9 @@
+export default {
+	html: `
+		<p style="--a: 0;"></p>
+		<p style="--b: false;"></p>
+		<p></p>
+		<p></p>
+		<p></p>
+	`
+};

packages/svelte/test/runtime/samples/inline-style-directive-falsy-value/main.svelte

@@ -0,0 +1,5 @@
+<p style:--a={0}></p>
+<p style:--b={false}></p>
+<p style:--c=""></p>
+<p style:--d={undefined}></p>
+<p style:--e={null}></p>