xmlimport breaks when UAVariable doesn't have value element

opcua/common/xmlparser.py

@@ -27,7 +27,7 @@ class NodeData(object):
         # variable
         self.datatype = None
         self.rank = -1  # check default value
-        self.value = []
+        self.value = None
         self.valuetype = None
         self.dimensions = None
         self.accesslevel = None

tests/custom_nodes.xml

@@ -39,5 +39,12 @@
     </Value>
   </UAVariable>
 
+  <UAVariable NodeId="i=30004" BrowseName="MyXMLVariableWithoutValue" DataType="String">
+    <References>
+      <Reference ReferenceType="HasTypeDefinition">i=69</Reference>
+      <Reference ReferenceType="Organizes" IsForward="false">i=30002</Reference>
+    </References>
+  </UAVariable>
+
 
 </UANodeSet>