[fix] do not collapse whitespace-only css vars (sveltejs#7303)

Fixes sveltejs#7152, see also sveltejs#7288

--foo:; used to be an invalid CSS custom property value, while -foo: ; was valid. By collapsing the whitespace in these declaration values, we were breaking scenarios where an empty custom property was desired. The spec was updated to trim whitespace and treat these values identically, but Chromium browsers still treat --foo; as invalid. This was recently fixed and will be released in Chrome 99, but this would still be a good fix to maintain backwards compatibility.

src/compiler/compile/css/Stylesheet.ts

@@ -145,6 +145,10 @@ class Declaration {
 			? this.node.value.children[0]
 			: this.node.value;
 
+		// Don't minify whitespace in custom properties, since some browsers (Chromium < 99)
+		// treat --foo: ; and --foo:; differently
+		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: ;--bar: !important}

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

@@ -7,4 +7,9 @@
 	div {
 		--test: 10;
 	}
+
+	div {
+		--foo: ;
+		--bar: !important;
+	}
 </style>