Can't export empty universal variables

[vvug]

This is pretty easy to reproduce:

~> # The XYZ env variable is not set:
~> env | grep XYZ
~> # Let's set it:
~> set -x XYZ
~> env | grep XYZ
XYZ=
~> # It's set. Now the same with a universal variable:
~> env | grep UVW
~> set -Ux UVW
~> env | grep UVW
~> # Not set!
~> fish --version
fish, version 3.0.2
~> echo $version
3.0.2

[zanchey]

The variable is set, but not exported:

> set -gx XYZ
> env | grep XYZ
XYZ=
> set --erase XYZ
> set -Ux XYZ
> env | grep XYZ
> set --show XYZ
$XYZ: not set in local scope
$XYZ: not set in global scope
$XYZ: set in universal scope, exported, with 0 elements