Throw if providers is empty (#146)

Author: marcoscaceres

index.html

@@ -301,6 +301,13 @@ <h3>
       </li>
       <li>[=Consume user activation=] of |window|.
       </li>
+      <li>Let |providers| be |options|'s
+        {{DigitalCredentialRequestOptions/digital}}'s
+        {{IdentityRequestProvider/providers}} member.
+        </li>
+        <li>If |providers| is empty, [=exception/throw=] a {{"TypeError"}}
+        {{DOMException}}.
+        </li>
       <li>
         <aside class="issue">
           Details of how to actually get the [=digital credential=] are