handle empty classes - fixes #978

Author: Rich-Harris

src/css/Selector.ts

@@ -229,6 +229,7 @@ function attributeMatches(node: Node, name: string, expectedValue: string, opera
 	const pattern = operators[operator](expectedValue, caseInsensitive ? 'i' : '');
 	const value = attr.value[0];
 
+	if (!value) return false;
 	if (value.type === 'Text') return pattern.test(value.data);
 
 	const possibleValues = new Set();

test/css/samples/empty-class/_config.js

@@ -0,0 +1,20 @@
+export default {
+	cascade: false,
+
+	warnings: [{
+		filename: "SvelteComponent.html",
+		message: "Unused CSS selector",
+		loc: {
+			line: 4,
+			column: 1
+		},
+		pos: 31,
+		frame: `
+			2:
+			3: <style>
+			4:   .x {
+			     ^
+			5:     color: red;
+			6:   }`
+	}]
+};

test/css/samples/empty-class/expected.css

@@ -0,0 +1,7 @@
+<div class=""></div>
+
+<style>
+	.x {
+		color: red;
+	}
+</style>