fixed up the attribute helper method

stdlib/ktSrc/dom/Dom.kt

@@ -190,9 +190,9 @@ fun Element.get(selector: String): List<Element> {
     }
 }
 
-/** Returns the attribute value or null if its not present */
-inline fun Element?.attribute(name: String): String? {
-    return this?.getAttribute(name)
+/** Returns the attribute value or empty string if its not present */
+inline fun Element.attribute(name: String): String {
+    return this.getAttribute(name) ?: ""
 }
 
 /** Returns the children of the element as a list */

testlib/test/dom/DomBuilderTest.kt

@@ -89,7 +89,7 @@ class DomBuilderTest() : TestSupport() {
         if (grandChild != null) {
             println("got element ${grandChild.toXmlString()} with text '${grandChild.text}`")
             assertEquals("Hello World!", grandChild.text)
-            assertEquals(" bar tiny", grandChild.attribute("class") ?: "")
+            assertEquals(" bar tiny", grandChild.attribute("class"))
 
             // test the classSet
             val classSet = grandChild.classSet