"$@" expansion when it is consists of only null strings

[Grisha Levit]

There are some what seem to be regressions (?) in bash-4.4 and
bash-5.0 regarding the handling of "$@" expansion when it consists
entirely of null strings.

The relevant commits are bash-20160722 [1], bash-20170331 [2],
bash-20190220 [3].

Using:
    $ n() { echo $#; }

Since bash-20160722:
    $ set -- ''
    $ n "$@"''
    0
    $ set -- '' ''
    n ''"$@"
    1
    n "$@"''
    1
    n ''"$@"''
    0

(and, since bash-20190220, also in the ${x+} PE form)

Since bash-20170331:

    $ set -- ''
    $ n ${_+''"$@"''}
    0
    $ n ${_+"$@" "$@"}
    1

On a related note, with $#=0, bash-20190220 fixed the expansion of
${_"$@"''} and ${''"$@"} to produce a single field but not so with:

    $ set --
    $ n ${_+''"$@"''}
    0

And these have always been the case, but I'm not sure if it should be so?

    $ set --
    $ n "${_+"$@"}"
    1
    $ n "${_+$@}"
    1

[1]: 
https://git.savannah.gnu.org/cgit/bash.git/commit/?h=devel&id=13eae87b0c5dbc31a7d352ce17c6817bb79b9226
[2]: 
https://git.savannah.gnu.org/cgit/bash.git/commit/?h=devel&id=7c4ef411b5f1a458c2908efe503e075e5a682040
[3]: 
https://git.savannah.gnu.org/cgit/bash.git/commit/?h=devel&id=9e48f856544da2d2cc95600f487e5b5bcefa0d85