spec: Remove remnant mention of InterfaceText allowing null

Follows-up 2c89726 which removed support for null values
in InterfaceText. The uses of InterfaceText that allow null
says so explicitly in their signature (e.g. Root.description
can be InterfaceText or null, whereas Set.label must be
InterfaceText and never null).

Change-Id: Ied1dbadd6b65d8b21244617d31a9fdd64edf8e2a
Merge "spec: Remove remnant mention of InterfaceText allowing null"