sensu plugin: fix compiler warning when build with --enable-debug

sensu plugin: fix compiler warning when build with --enable-debug

In file included from src/write_sensu.c:31:
src/write_sensu.c: In function ‘sensu_config’:
./src/daemon/plugin.h:398:20: warning: ‘%s’ directive argument is null [-Wformat-overflow=]
  398 | #define DEBUG(...) plugin_log(LOG_DEBUG, __VA_ARGS__)
      |                    ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
src/write_sensu.c:1227:7: note: in expansion of macro ‘DEBUG’
 1227 |       DEBUG("write_sensu plugin: Got tag: %s", tmp);
      |       ^~~~~
src/write_sensu.c:1227:43: note: format string is defined here
 1227 |       DEBUG("write_sensu plugin: Got tag: %s", tmp);
      |