NA in mdv column not filled properly

```r
ev(amt = 100) %>% 
  as_data_set() %>% 
  obs_rows(time = 1, cmt = 1)
# A tibble: 2 x 7
     ID  time  evid   cmt   amt    DV   mdv
  <int> <dbl> <int> <int> <dbl> <dbl> <int>
1     1     0     1     1   100    NA    NA
2     1     1     0     1     0    NA     1

```