Appending to an fd doesn't really make sense, but we allowed the
syntax previously and it was actually used.
It's not too harmful to allow it, so let's just do that again.
For the record: Zsh also allows it, bash doesn't.
Fixes#6614
This was previously required so that, if there was a redirection to a
file, we would fork a process to create the file even if there was no
output. For example `echo -n >/tmp/file.txt` would have to create
file.txt even though it would be empty.
However now we open the file before fork, so we no longer need special
logic around this.
