Bug 1617546 - Provide operator== for mozilla::Nothing. r=froydnj

Differential Revision: https://phabricator.services.mozilla.com/D63842

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

mfbt/Maybe.h

@@ -33,6 +33,10 @@ namespace mozilla {
 
 struct Nothing {};
 
+inline constexpr bool operator==(const Nothing&, const Nothing&) {
+  return true;
+}
+
 namespace detail {
 
 // You would think that poisoning Maybe instances could just be a call