[Rd] `as.data.frame.matrix()` can produce a data frame without a `names` attribute

[Davis Vaughan]

Hi all,

I recently learned that it is possible for `as.data.frame.matrix()` to
produce a data frame with 0 columns that is also entirely missing a
`names` attribute, and I think this is a bug:

```
# No `names`, weird!
attributes(as.data.frame(matrix(nrow = 0, ncol = 0)))
#> $class
#> [1] "data.frame"
#>
#> $row.names
#> integer(0)

# This is what I expected
attributes(data.frame())
#> $names
#> character(0)
#>
#> $row.names
#> integer(0)
#>
#> $class
#> [1] "data.frame"
```

In my experience, 0 column data frames should probably still have a
`names` attribute, and it should be set to `character()`. Some
evidence to support my theory is that OOB subsetting doesn't give the
intended error with this weird data frame:

```
# Good OOB error
df <- data.frame()
df[1]
#> Error in `[.data.frame`(df, 1): undefined columns selected

# This is weird!
df <- as.data.frame(matrix(nrow = 0, ncol = 0))
df[1]
#> NULL
#> <0 rows> (or 0-length row.names)
```

The one exception to requiring a `names` attribute that I can think of
is `as.data.frame(optional = TRUE)`, mostly for internal use by
`data.frame()` on each of the columns, but that doesn't seem to apply
here.

Thanks,
Davis Vaughan