optional: Fix compilation on XLClang 16.1.0

The compiler rejects initialization with `= {}`:

    error: chosen constructor is explicit in copy-initialization

Use just `{}` instead.

include/dap/optional.h

@@ -75,7 +75,7 @@ class optional {
   inline T& operator*();
 
  private:
-  T val = {};
+  T val{};
   bool set = false;
 };