Fix crash with invalid parameter combination and add coverage to WPT

Differential Revision: https://phabricator.services.mozilla.com/D220258

bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=1914853
gecko-commit: 40d0f1c943cbcf1174d99627d4b20d50c3bdc9b1
gecko-reviewers: anti-tracking-reviewers, timhuang

Author: bvandersloot-mozilla

fedcm/lfedcm-identity.create-store-collect.tentative.sub.https.html

@@ -67,6 +67,16 @@
 }, "Basic identity credential store suceeds");
 
 
+promise_test(async t => {
+
+  let promise4 = navigator.credentials.get({identity: {providers: []}});
+  assert_true(promise4 instanceof Promise, "The return value of get must be a promise.");
+  let result4 = await promise4;
+  assert_equals(result4, null, "A credential is not returned from the CredentialContainer when a request with no providers is given");
+
+}, "Collecting from no providers yields a promise with null resolution");
+
+
 promise_test(async t => {
   let initData = {
     id : "wpt-pick-me",