Fix NPE on empty .xsd document

Added some null checks to prevent the NPE occuring.

Fixes eclipse-lemminx#684

org.eclipse.lemminx/src/main/java/org/eclipse/lemminx/extensions/xsd/utils/XSDUtils.java

@@ -289,6 +289,9 @@ public static void searchXSOriginAttributes(DOMNode targetNode, BiConsumer<DOMAt
 	 */
 	private static List<DOMAttr> getTargetAttrs(DOMNode referencedNode) {
 		List<DOMAttr> referencedNodes = new ArrayList<>();
+		if (referencedNode == null) {
+			return referencedNodes;
+		}
 		Document document = referencedNode.getOwnerDocument();
 		switch (referencedNode.getNodeType()) {
 		case Node.ATTRIBUTE_NODE:
@@ -302,6 +305,9 @@ private static List<DOMAttr> getTargetAttrs(DOMNode referencedNode) {
 			// The referenced node is the DOM document, collect all attributes
 			// xs:complexType/@name, xs:simpleType/@name, xs:element/@name, xs:group/@name
 			// which can be referenced
+			if (document.getDocumentElement() == null) {
+				break;
+			}
 			NodeList nodes = document.getDocumentElement().getChildNodes();
 			for (int i = 0; i < nodes.getLength(); i++) {
 				Node n = nodes.item(i);