Merge pull request #42 from josemduarte/master

Bugfix: the null strings shouldn't be converted to ""

mmtf-serialization/src/main/java/org/rcsb/mmtf/serialization/quickmessagepackdeserialization/ObjectTree.java

@@ -31,7 +31,7 @@ public Object get(String s) {
 	public String getString(String s) {
 		Object o = root.get(s);
 		if (o == null) {
-			return "";
+			return null;
 		} else {
 			return (String) root.get(s);
 		}