Join NA_character_ to factor broken in dev

Causes VIM to fail its tests.
```R
> DT = data.table(fac = as.factor(c("a",NA,"b")), v=1:3, key="fac")
> DT
      fac     v
   <fctr> <int>
1:   <NA>     2
2:      a     1
3:      b     3

> DT[.(NA_character_)]   # correct in 1.12.2
      fac     v
   <fctr> <int>
1:   <NA>     2

> DT[.(NA_character_)]   # incorrect in dev
      fac     v
   <char> <int>
1:   <NA>    NA
```