monitor: fix use of plain integer as NULL pointer, spotted by Sparse

Signed-off-by: Blue Swirl <blauwirbel@gmail.com>

monitor.c

diff --git a/monitor.c b/monitor.c
index 689a853..ba76f34 100644
--- a/monitor.c
+++ b/monitor.c
@@ -3757,7 +3757,7 @@
     const char *p;
     CmdArgs cmd_args;
 
-    if (cmd->args_type == '\0') {
+    if (cmd->args_type == NULL) {
         return (qdict_size(args) == 0 ? 0 : -1);
     }