A couple more nullopt returns (#15422)

src/openrct2/core/String.hpp

@@ -169,7 +169,7 @@ namespace String
         {
             return { 4 };
         }
-        return {};
+        return std::nullopt;
     }
 
     /**

src/openrct2/object/ObjectManager.cpp

@@ -419,7 +419,7 @@ private:
                 return static_cast<int32_t>(i);
             }
         }
-        return {};
+        return std::nullopt;
     }
 
     size_t GetLoadedObjectIndex(const Object* object)

src/openrct2/rct12/RCT12.cpp

@@ -1410,5 +1410,5 @@ std::optional<uint8_t> GetStyleFromMusicIdentifier(std::string_view identifier)
     {
         return std::distance(std::begin(_musicStyles), it);
     }
-    return {};
+    return std::nullopt;
 }