[Java] Restore belt and braces null check.

Author: mikeb01

sbe-tool/src/main/java/uk/co/real_logic/sbe/xml/XmlSchemaParser.java

@@ -364,10 +364,10 @@ public static String getAttributeValueOrNull(final Node elementNode, final Strin
  }
 
  final NamedNodeMap attributes = elementNode.getAttributes();
-// if (null == attributes)
-// {
-// return null;
-// }
+ if (null == attributes)
+ {
+ return null;
+ }
 
  final Node attrNode = attributes.getNamedItemNS(null, attrName);
  if (null == attrNode)