AK: Implement HashMap::ensure(key) in terms of ensure(key, callback)

In cases where the key is not present, the previous implementation
would do three lookups, when only one should be necessary, as a bucket
must exist after the method returns.

AK/HashMap.h

@@ -265,22 +265,17 @@ public:
         return take(begin()->key).release_value();
     }
 
-    V& ensure(K const& key)
-    {
-        auto it = find(key);
-        if (it != end())
-            return it->value;
-        auto result = set(key, V());
-        VERIFY(result == HashSetResult::InsertedNewEntry);
-        return find(key)->value;
-    }
-
     template<typename Callback>
     V& ensure(K const& key, Callback initialization_callback, HashSetExistingEntryBehavior existing_entry_behavior = HashSetExistingEntryBehavior::Keep)
     {
         return m_table.ensure(KeyTraits::hash(key), [&](auto& entry) { return KeyTraits::equals(entry.key, key); }, [&] -> Entry { return { key, initialization_callback() }; }, existing_entry_behavior).value;
     }
 
+    V& ensure(K const& key)
+    {
+        return ensure(key, [] { return V(); });
+    }
+
     template<typename Callback>
     ErrorOr<V> try_ensure(K const& key, Callback initialization_callback)
     {