Apply RFC 8264 enforcement only to non-empty strings (#2073)

* Disallow empty strings

* Update index.bs

Co-authored-by: Matthew Miller <matthew@millerti.me>

* Apply RFC 8264 enforcement only to non-empty strings

---------

Co-authored-by: Matthew Miller <matthew@millerti.me>

index.bs

@@ -3324,14 +3324,15 @@ credential.
 
         - [=[RPS]=] SHOULD perform enforcement, as prescribed in Section 2.3 of
             [[!RFC8266]] for the Nickname Profile of the PRECIS FreeformClass [[!RFC8264]],
-            when setting {{PublicKeyCredentialUserEntity/displayName}}'s value, or displaying the value to the user.
+            when setting {{PublicKeyCredentialUserEntity/displayName}}'s value to a non-empty string,
+            or displaying a non-empty value to the user.
 
         - This string MAY contain language and direction metadata. [=[RPS]=] SHOULD consider providing this information. See [[#sctn-strings-langdir]] about how this metadata is encoded.
 
         - [=Clients=] SHOULD perform enforcement, as prescribed in Section 2.3 of
             [[!RFC8266]] for the Nickname Profile of the PRECIS FreeformClass [[!RFC8264]],
-            on {{PublicKeyCredentialUserEntity/displayName}}'s value prior to displaying the value to the user or
-            including the value as a parameter of the [=authenticatorMakeCredential=] operation.
+            on {{PublicKeyCredentialUserEntity/displayName}}'s value prior to displaying a non-empty value to the user or
+            including a non-empty value as a parameter of the [=authenticatorMakeCredential=] operation.
 
         When [=clients=], [=client platforms=], or [=authenticators=] display a {{PublicKeyCredentialUserEntity/displayName}}'s value, they should always use UI elements to provide a clear boundary around the displayed value, and not allow overflow into other elements [[css-overflow-3]].