Fix buglet in setPrioritySignals. (#712)

The second argument can be omitted/set to null/undefined to erase
mapping for a key. The actual handling logic was already handling
that, but we were not accepting this in the signature, or permitting in
the infra rep.

Co-authored-by: Maks Orlovich <[email protected]>

spec.bs

@@ -2299,7 +2299,7 @@ interface InterestGroupBiddingScriptRunnerGlobalScope
         : InterestGroupScriptRunnerGlobalScope {
   boolean setBid(optional GenerateBidOutput generateBidOutput = {});
   undefined setPriority(double priority);
-  undefined setPrioritySignalsOverride(DOMString key, double priority);
+  undefined setPrioritySignalsOverride(DOMString key, optional double? priority);
 };
 
 dictionary AdRender {
@@ -2328,7 +2328,7 @@ Each {{InterestGroupBiddingScriptRunnerGlobalScope}} has a
 : <dfn>priority</dfn>
 :: Null or a {{double}}
 : <dfn>priority signals</dfn>
-:: An [=ordered map=] whose [=map/keys=] are [=strings=] and whose [=map/values=] are {{double}}
+:: An [=ordered map=] whose [=map/keys=] are [=strings=] and whose [=map/values=] are {{double}} or null.
 : <dfn>interest group</dfn>
 :: An [=interest group=]
 : <dfn>expected currency</dfn>