macos: fix Xcode 26 build compatibility (#4226)

platform/macos/src/MLNMapView.mm

@@ -24,6 +24,8 @@
 #import "MLNPolyline.h"
 #import "MLNSettings.h"
 
+#include <optional>
+
 #import <mbgl/annotation/annotation.hpp>
 #import <mbgl/map/camera.hpp>
 #import <mbgl/map/map.hpp>
@@ -317,8 +319,9 @@ public:
 
   MLNRendererConfiguration *config = [MLNRendererConfiguration currentConfiguration];
 
-  auto localFontFamilyName =
-      config.localFontFamilyName ? std::string(config.localFontFamilyName.UTF8String) : nullptr;
+  std::optional<std::string> localFontFamilyName =
+      config.localFontFamilyName ? std::optional(std::string(config.localFontFamilyName.UTF8String))
+                                 : std::nullopt;
   auto renderer = std::make_unique<mbgl::Renderer>(_mbglView->getRendererBackend(),
                                                    config.scaleFactor, localFontFamilyName);
   BOOL enableCrossSourceCollisions = !config.perSourceCollisions;