Explicitly clarify the intent behind optional and required fields

Author: kpshek

docs/specification/1.0.md

@@ -7,6 +7,10 @@
 
 The CDS Hooks specification describes the RESTful APIs and interactions to integrate Clinical Decision Support (CDS) between CDS Clients (typically Electronic Health Record Systems (EHRs) or other health information systems) and CDS Services. All data exchanged through the RESTful APIs MUST be sent and received as [JSON](https://tools.ietf.org/html/rfc8259) structures, and MUST be transmitted over channels secured using the Hypertext Transfer Protocol (HTTP) over Transport Layer Security (TLS), also known as HTTPS and defined in [RFC2818](https://tools.ietf.org/html/rfc2818).
 
+Unless otherwise specified, JSON attributes SHALL NOT be null. If a JSON attribute is defined with an optionality of OPTIONAL, but does not have a value, implementers MUST omit it. For instance, OPTIONAL JSON string and array attributes should be omitted rather than having a null or empty value.
+
+Unless otherwise specified, JSON string or URL attributes that have an optionality of REQUIRED MAY NOT have empty values (those without any characters or just whitespace characters).
+
 ### Conformance Language
 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this specification are to be interpreted as described in [RFC2119](https://tools.ietf.org/html/rfc2119).