Fix problem with an empty parameter in set_base_dir

Author: yhirose

httplib.h

@@ -622,7 +622,7 @@ class Server {
   Server &Options(const std::string &pattern, Handler handler);
 
   bool set_base_dir(const std::string &dir,
-                    const std::string &mount_point = nullptr);
+                    const std::string &mount_point = std::string());
   bool set_mount_point(const std::string &mount_point, const std::string &dir,
                        Headers headers = Headers());
   bool remove_mount_point(const std::string &mount_point);