fix: do not collapse whitespace-only css vars

src/compiler/compile/css/Stylesheet.ts

@@ -145,6 +145,8 @@ class Declaration {
 			? this.node.value.children[0]
 			: this.node.value;
 
+		if (first.type === 'Raw' && /^\s+$/.test(first.value)) return;
+
 		let start = first.start;
 		while (/\s/.test(code.original[start])) start += 1;
 

test/css/samples/css-vars/expected.css

@@ -1 +1 @@
-:root{--root-test:20}div.svelte-xyz{--test:10}
+:root{--root-test:20}div.svelte-xyz{--test:10}div.svelte-xyz{--foo: }

test/css/samples/css-vars/input.svelte

@@ -7,4 +7,8 @@
 	div {
 		--test: 10;
 	}
+
+	div {
+		--foo: ;
+	}
 </style>