fix: drop style attribute when value normalises to empty

Previously the client kept an empty `style=""` for empty-string values
while SSR dropped the attribute, which is a hydration mismatch and now
also affects every object/array input that filters down to nothing
(`style={false}`, `style={{}}`, `style={[null, false]}`). Coerce empty
to `null` in `to_style` so client and SSR agree, matching what the
directive path already did and what `to_class` does for `class={''}`.

Two existing tests asserted `<div style="">` for `component.style = ''`
as an incidental side effect; updated to expect the attribute dropped.

packages/svelte/src/internal/shared/attributes.js

@@ -261,5 +261,7 @@ export function to_style(value, styles) {
 		return new_style === '' ? null : new_style;
 	}
 
-	return value == null ? null : String(value);
+	// Empty results drop the attribute entirely so that client and SSR agree
+	// (the directive path above already returns `null` for empty output).
+	return value == null || value === '' ? null : String(value);
 }

packages/svelte/tests/runtime-runes/samples/async-fork-attributes/_config.js

@@ -12,8 +12,8 @@ export default test({
 			`
 			<button>fork</button>
 			<button>commit</button>
-			<p style="">foo</p>
-			<p style="">foo</p>
+			<p>foo</p>
+			<p>foo</p>
 			<p>foo</p>
 		`
 		);
@@ -51,8 +51,8 @@ export default test({
 			`
 			<button>fork</button>
 			<button>commit</button>
-			<p style="">foo</p>
-			<p style="">foo</p>
+			<p>foo</p>
+			<p>foo</p>
 			<p>foo</p>
 		`
 		);

packages/svelte/tests/runtime-runes/samples/style-update/_config.js

@@ -37,14 +37,16 @@ export default test({
 		component.style = '';
 		flushSync();
 
+		// empty results drop the attribute on both client and SSR (was previously
+		// `<div style=""></div>` on the client only — a hydration mismatch)
 		assert.htmlEqual(
 			target.innerHTML,
 			`
-			<div style=""></div>
-			<div style=""></div>
+			<div></div>
+			<div></div>
 
-			<custom-element style=""></custom-element>
-			<custom-element style=""></custom-element>
+			<custom-element></custom-element>
+			<custom-element></custom-element>
 			`
 		);