Fix optional vpp args handling

Signed-off-by: Dave Wallace <dwallacelf@gmail.com>

vppcfg

@@ -199,9 +199,9 @@ def main():
     )
 
     opt_kwargs = {}
-    if "vpp_json_dir" in args:
+    if "vpp_json_dir" in args and args.vpp_json_dir is not None:
         opt_kwargs["vpp_json_dir"] = args.vpp_json_dir
-    if "vpp_api_socket" in args:
+    if "vpp_api_socket" in args and args.vpp_api_socket is not None:
         opt_kwargs["vpp_api_socket"] = args.vpp_api_socket
 
     if args.command == "dump":