Allow empty object types

spec/Section 3 -- Type System.md

@@ -150,7 +150,7 @@ type.
 
 Similarly, the {`subscription`} _root operation type_ is also optional; if it is
 not provided, the service does not support subscriptions. If it is provided, it
-must be an Object type.
+must be an Object type and must define at least one field.
 
 The {`query`}, {`mutation`}, and {`subscription`} root types must all be
 different types if provided.
@@ -922,7 +922,7 @@ Objects are never valid inputs.
 Object types have the potential to be invalid if incorrectly defined. This set
 of rules must be adhered to by every Object type in a GraphQL schema.
 
-1. An Object type must define one or more fields.
+1. An Object type must define a list of fields. The list may be empty.
 2. For each field of an Object type:
    1. The field must have a unique name within that Object type; no two fields
       may share the same name.