use blank string instead of NULL

git-svn-id: http://svn.freeswitch.org/svn/freeswitch/trunk@12236 d0543943-73ff-0310-b7d9-9358b9ac24b2

libs/esl/src/esl.c

@@ -471,7 +471,7 @@ ESL_DECLARE(esl_status_t) esl_execute(esl_handle_t *handle, const char *app, con
 	}
 
 	snprintf(send_buf, sizeof(send_buf), "%s\ncall-command: execute\n%s%s%s%s\n", 
-			 cmd_buf, app_buf, arg_buf, handle->event_lock ? el_buf : NULL, handle->blocking_execute ? bl_buf : NULL);
+			 cmd_buf, app_buf, arg_buf, handle->event_lock ? el_buf : "", handle->blocking_execute ? bl_buf : "");
 
 	return esl_send_recv(handle, send_buf);
 }