x[NA_integer64_] throws warning from min()

```r
as.integer64(1L)[NA_integer_]
# integer64
# [1] <NA>
# Warning message:
# In min(i, na.rm = TRUE) : no non-missing arguments to min; returning Inf
```

c.f.

```r
1L[NA_integer_]
# [1] NA
```

(this is probably not a regression as it's from the ignored duplicate definition of `[` that I'm switching to as part of #176)