Stack imbalance with NULLs in rbindlist when use.names, fill=TRUE

The following fails with a stack imbalance:

```
rbindlist(replicate(4,rbindlist(replicate(47, NULL), use.names=TRUE, fill=TRUE)), use.names=TRUE, fill=TRUE)
## Warning: stack imbalance in 'lapply', 13 then 15
## Null data.table (0 rows and 0 cols)
```

Unfortunately this is in a routine where data may or may not be present, so I need to handle it.
