Add default values to registration info (WICG#1425)

index.bs

@@ -576,9 +576,9 @@ therefore exceed the intended rate limits at will.
 A registration info is a [=struct=] with the following items:
 
 <dl dfn-for="registration info">
-: <dfn>preferred platform</dfn>
+: <dfn>preferred platform</dfn> (default null)
 :: Null or a [=registrar=].
-: <dfn>report header errors</dfn>
+: <dfn>report header errors</dfn> (default false)
 :: A [=boolean=].
 
 </dl>
@@ -1780,7 +1780,7 @@ returned to avoid data loss from the optional debug reporting feature.
 To <dfn>get registration info from a header list</dfn> given a
 [=header list=] |headers|:
 
-1. If |headers| does not [=header list/contain=] "`Attribution-Reporting-Info`", return null.
+1. If |headers| does not [=header list/contain=] "`Attribution-Reporting-Info`", return a new [=registration info=].
 1. Let |map| be the result of
     [=header list/get a structured field value|getting=] "`Attribution-Reporting-Info`" from |headers|
     with a type of "`dictionary`".