Fix undefined declaration value (fix #553)

lib/single-file/single-file-core.js

@@ -1716,12 +1716,16 @@ this.singlefile.lib.core = this.singlefile.lib.core || (() => {
 
 		static async processStyle(declarations, baseURI, options, cssVariables, batchRequest) {
 			await Promise.all(declarations.map(async declaration => {
-				let children = declaration.value.children;
-				if (!children && declaration.value && declaration.value.type == "Raw") {
-					try {
-						children = cssTree.parse(declaration.value.value, { context: "value" }).children;
-					} catch (error) {
-						// ignored
+				let children;
+				if (declaration.value) {
+					if (declaration.value.children) {
+						children = declaration.value.children;
+					} else if (declaration.value.type == "Raw") {
+						try {
+							children = cssTree.parse(declaration.value.value, { context: "value" }).children;
+						} catch (error) {
+							// ignored
+						}
 					}
 				}
 				if (declaration.type == "Declaration" && children) {