Fix mappath config default

Author: manics

jupyter_server_proxy/config.py

@@ -106,7 +106,7 @@ def make_server_process(name, server_process_config):
         timeout=server_process_config.get('timeout', 5),
         absolute_url=server_process_config.get('absolute_url', False),
         port=server_process_config.get('port', 0),
-        mappath=server_process_config.get('mappath', ''),
+        mappath=server_process_config.get('mappath', {}),
         launcher_entry=LauncherEntry(
             enabled=le.get('enabled', True),
             icon_path=le.get('icon_path'),