Problem: On some systems `stderr` can be disabled. This results in not
usable `vim.pack` since it asserted `stderr` to be non-nil.
Solution: Stop asserting non-nil `stderr`. The downside is that
potential errors are not shown, but this is intentional since `stderr`
is disabled on system level.
Still assert non-nil `stdout` as its output is important for
`vim.pack` to actually do its job. Disabled `stdout` is not something
that can work with `vim.pack`.
(cherry picked from commit 25d33dd12b)
f533a17958