adding clarification about null and empty stringx2

Author: gbinal

Diff for: schema.md

@@ -33,6 +33,8 @@ The [Implementation Guidance](/implementation-guide/) available as a part of Pro
 
 [JSON](http://www.json.org) is a lightweight data-exchange format that is very easy to read, parse and generate.  Based on a subset of the JavaScript programming language, JSON is a text format that is optimized for data interchange.  JSON is built on two structures: (1) a collection of name/value pairs; and (2) an ordered list of values.  
 
+Where optional fields are included in a catalog file but are unpopulated, they may be represented by a `null` value.  They should not be represented by an empty string (`""`).  
+
 Links to downloadable examples of metadata files developed in this and other formats in [the metadata resources](/metadata-resources/).  Tools to help agencies produce and maintain their data inventories are [available on GitHub](http://www.github.com/project-open-data) and hosted at [Labs.Data.gov](http://labs.data.gov).