Bug 1601483 - Avoid HashTable lookups when the table is empty r=froydnj

Differential Revision: https://phabricator.services.mozilla.com/D55882

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

mfbt/HashTable.h

@@ -2056,7 +2056,7 @@ class HashTable : private AllocPolicy {
   }
 
   MOZ_ALWAYS_INLINE Ptr readonlyThreadsafeLookup(const Lookup& aLookup) const {
-    if (!mTable || !HasHash<HashPolicy>(aLookup)) {
+    if (empty() || !HasHash<HashPolicy>(aLookup)) {
       return Ptr();
     }
     HashNumber keyHash = prepareHash(aLookup);