Bug 1486119 - Pocket: Replace getSystemPrincipal opening new tab with createNullPrincipal r=jkt

Opening a new tab with a hardcoded link with the system utility openWebLinkI
was erroring when passing in getSystemPrincipal. Updating code to pass in a
createNullPrincipal.

Differential Revision: https://phabricator.services.mozilla.com/D4262

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

browser/extensions/pocket/content/main.js

@@ -150,7 +150,7 @@ var pktUI = (function() {
         if (pktApi.getSignupPanelTabTestVariant() == "v2") {
             let site = Services.prefs.getCharPref("extensions.pocket.site");
             openTabWithUrl("https://" + site + "/firefox_learnmore?s=ffi&t=autoredirect&tv=page_learnmore&src=ff_ext",
-                           Services.scriptSecurityManager.getSystemPrincipal());
+                           Services.scriptSecurityManager.createNullPrincipal({}));
 
             // force the panel closed before it opens
             getPanel().hidePopup();