Bug 1572401 - Don't display "{$name}" for waiting cache ID. r=perry

There was a falsey check that was treating an empty string to interpolate
as equivalent to undefined having been passed, erroneously displaying the
templating string instead of replacing it with an empty string.

Depends on D66150

Differential Revision: https://phabricator.services.mozilla.com/D66151

--HG--
extra : moz-landing-system : lando

toolkit/content/aboutServiceWorkers.js

@@ -81,7 +81,9 @@ async function display(info, pushService) {
     let bold = document.createElement("strong");
     bold.setAttribute("data-l10n-name", "item-label");
     item.appendChild(bold);
-    if (!value) {
+    // Falsey values like "" are still valid values, so check exactly against
+    // undefined for the cases where the caller did not provide any value.
+    if (value === undefined) {
       document.l10n.setAttributes(item, l10nId);
     } else if (makeLink) {
       let link = document.createElement("a");