oc: Modify read method to return empty string instead of nil

Changed the `read` method in `mill_engine.mm` to return an empty NSString (`@""`)
instead of `nil`. This addresses the issue where `nil` was being returned from a
method expected to return a non-null value, ensuring compliance with the
method's contract and preventing potential runtime errors.

src/ui/flutter_app/command/mill_engine.mm

@@ -94,7 +94,7 @@ -(NSString *) read {
     bool got_response = channel->popupResponse(buffer);
 
     if (!got_response) {
-        return nil;
+        return @"";
     }
 
     NSString *line = [NSString stringWithFormat:@"%s", buffer];