chore(alt-space-value): use virtualNode instead of node (dequelabs#1924)

Author: WilcoFiers

lib/checks/label/alt-space-value.js

@@ -1,2 +1,3 @@
-const validAttrValue = /^\s+$/.test(node.getAttribute('alt'));
-return node.hasAttribute('alt') && validAttrValue;
+const alt = virtualNode.attr('alt');
+const isOnlySpace = /^\s+$/;
+return typeof alt === 'string' && isOnlySpace.test(alt);