Default to ES256 and RS256 if pubKeyCredParams is empty (#1387)

Default to algorithm -7 ("ES256") and -257 ("RS256") when
options.pubKeyCredParams is empty.

Fixes issue 1383.

index.bs

@@ -1490,16 +1490,25 @@ a numbered step. If outdented, it (today) is rendered as a bullet in the midst o
 1. Let |credTypesAndPubKeyAlgs| be a new [=list=] whose [=list/items=] are pairs of {{PublicKeyCredentialType}} and
     a {{COSEAlgorithmIdentifier}}.
 
-1. [=list/For each=] |current| of <code>|options|.{{PublicKeyCredentialCreationOptions/pubKeyCredParams}}</code>:
-
-    1. If <code>|current|.{{PublicKeyCredentialParameters/type}}</code> does not contain a {{PublicKeyCredentialType}} supported
-        by this implementation, then [=continue=].
-    1. Let |alg| be <code>|current|.{{PublicKeyCredentialParameters/alg}}</code>.
-    1. [=list/Append=] the pair of <code>|current|.{{PublicKeyCredentialParameters/type}}</code> and |alg| to
-        |credTypesAndPubKeyAlgs|.
-
-1. If |credTypesAndPubKeyAlgs| [=list/is empty=] and <code>|options|.{{PublicKeyCredentialCreationOptions/pubKeyCredParams}}</code>
-    [=list/is not empty=], return a {{DOMException}} whose name is "{{NotSupportedError}}", and terminate this algorithm.
+1. If <code>|options|.{{PublicKeyCredentialCreationOptions/pubKeyCredParams}}</code>'s [=list/size=]
+    <dl class="switch">
+        :   is zero
+        ::  [=list/Append=] the following pairs of {{PublicKeyCredentialType}} and {{COSEAlgorithmIdentifier}} values to |credTypesAndPubKeyAlgs|:
+             * {{public-key}} and <code>-7</code> ("ES256").
+             * {{public-key}} and <code>-257</code> ("RS256").
+
+        :   is non-zero
+        ::  [=list/For each=] |current| of <code>|options|.{{PublicKeyCredentialCreationOptions/pubKeyCredParams}}</code>:
+
+            1. If <code>|current|.{{PublicKeyCredentialParameters/type}}</code> does not contain a {{PublicKeyCredentialType}} supported
+                by this implementation, then [=continue=].
+            1. Let |alg| be <code>|current|.{{PublicKeyCredentialParameters/alg}}</code>.
+            1. [=list/Append=] the pair of <code>|current|.{{PublicKeyCredentialParameters/type}}</code> and |alg| to
+                |credTypesAndPubKeyAlgs|.
+
+            If |credTypesAndPubKeyAlgs| [=list/is empty=], return a {{DOMException}} whose name is
+               "{{NotSupportedError}}", and terminate this algorithm.
+    </dl>
 
 1. Let |clientExtensions| be a new [=map=] and let |authenticatorExtensions| be a new [=map=].