[drmemtrace] -record_syscall fails to record syscalls with zero args

Specifying 0 args for a syscall in `-record_syscall` to drmemtrace fails with the following assert:

```
$ ./bin64/drrun -t drmemtrace -record_syscall "1|0" -- suite/tests/bin/simple_app
...
ASSERT FAILURE: dynamorio/ext/drcontainers/hashtable.c:365: payload != NULL (hashtable_add_replace internal error)
```

Of course `-record_syscall "1|1"` in the above works fine.