throw 'x' must be a character vector when calling  .Internal(iconv(NULL,"","","",TRUE,FALSE))

in function iconvlist

```R
    .Internal(iconv(NULL, "", "", "", TRUE, FALSE))
```

GNU-R output:
```R
NULL
```
fastR output:
```
Error: 'x' must be a character vector
```

this can be triggered by 

```R
library(readr)
read_lines('temp.text')
```