Use optional for the engine path

- A small quality of file change is to change the type of engine path
  from a string to an optional string, skips the binary directory
  lookup, which is commonly disabled by people who create wasm builds or
  include stockfish as a library.

closes https://github.com/official-stockfish/Stockfish/pull/5575

No functional change

src/engine.h

@@ -47,7 +47,7 @@ class Engine {
     using InfoFull  = Search::InfoFull;
     using InfoIter  = Search::InfoIteration;
 
-    Engine(std::string path = "");
+    Engine(std::optional<std::string> path = std::nullopt);
 
     // Cannot be movable due to components holding backreferences to fields
     Engine(const Engine&)            = delete;