[Rd] install packages with missing pkg argument

[Ant F]

Dear all,

The help for `?install.packages` decribes, in the `pkg` argument
description :

> If this is missing, a listbox of available packages is presented where
possible in an interactive R session.

In fact running it with a missing argument triggers an error :

install.packages()
> Error in install.packages : argument "pkgs" is missing, with no default

What however doesn't trigger an error is callinginstall.packages on a zero
length character :

install.packages(character(0))

On my colleague's R 3.5.1 windows installation it shows the listbox of
available packages, on my 3.6.0 installation it pauses for a couple seconds
and doesn't do anything.

A character vector of length zero is what you get when you compute an empty
`setdiff` or `intersection`, so it was very surprising to us to see
something popup where we were expecting a vector of missing packages to be
installed (or none if there was no missing package).

I believe having the function work as advertised with a proper missing
argument, and having it do nothing silently when the argument is of length
zero, would make more sense.

Best regards,

Antoine

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