bug#65475: 29.1; package-selected-packages variable is not updated when the last package is deleted

[Stefan Kangas]

> The issue here is that `package--save-selected-packages' only updates
> the value of `package-selected-packages', if the new value is non-nil,
> presumably because the VALUE argument is optional,

This was added in d0a5162fd825, fixing Bug#20855.  Personally, I'm not
a huge fan of that fix, as nil is clearly a valid (if infrequent)
value here.

Could something like this work?

diff --git a/lisp/emacs-lisp/package.el b/lisp/emacs-lisp/package.el
index e1172d69bf0..9f97f950e64 100644
--- a/lisp/emacs-lisp/package.el
+++ b/lisp/emacs-lisp/package.el
@@ -1982,7 +1982,7 @@ package--find-non-dependencies

 (defun package--save-selected-packages (&optional value)
   "Set and save `package-selected-packages' to VALUE."
-  (when value
+  (when (or value after-init-time)
     (setq package-selected-packages value))
   (if after-init-time
       (customize-save-variable 'package-selected-packages
package-selected-packages)