Fix issue with whitespace nodes

Author: Lars Klein

src/DocumentFormat.OpenXml/OpenXmlPartReader.cs

@@ -451,6 +451,12 @@ private bool MoveToNextElement()
                 case ElementState.MiscNode:
                     // cursor is end element, pop stack
                     _elementStack.Pop();
+                    if (_elementStack.Count == 0)
+                    {
+                        _elementState = ElementState.EOF;
+                        return false;
+                    }
+
                     break;
 
                 case ElementState.LeafStart:
@@ -481,6 +487,10 @@ private bool MoveToNextElement()
 
             _elementState = ElementState.Null;
 
+            while (_xmlReader.NodeType == XmlNodeType.Whitespace) {
+                _xmlReader.Read();
+            }
+
             if (_xmlReader.EOF || !_xmlReader.Read())
             {
                 _elementState = ElementState.EOF;