readr::read_file() and readr::read_file_raw() crash R session when given an empty raw vector

When passing an empty raw vector to readr::read_file() or readr::read_file_raw(), the R session immediately stops.

```r
print(R.version.string)
#> [1] "R version 4.5.1 (2025-06-13 ucrt)"
packageVersion("readr")
#> [1] '2.1.5'
x = raw()
readr::read_file(x) # <-- Crash
```

Thanks for your time and for maintaining readr.