xfconf: remove properties with null values

[pjstadig]

Description

In some cases, I'd like to remove xfconf properties. For example, if you set a keyboard shortcut's value to an empty string:

xfconf.settings.xfce4-keyboard-shortcuts."commands/custom/<Alt><Super>s" = "";

Rather than removing the property, this will cause an error. A property must be removed with xfconf-query -r ....

I had considered adding a separate list of properties for removal:

xfconf.unsettings.xfce4-keyboard-shortcuts = [ "commands/custom/<Alt><Super>s" ]

but that would allow for both setting and unsetting the same property and doesn't allow for defaults vs. overrides, etc.

Checklist

• Change is backwards compatible.

• Code formatted with ./format.

• Code tested through nix-shell --pure tests -A run.all or nix develop --ignore-environment .#all using Flakes.

• Test cases updated/added.

• Commit messages are formatted like

  {component}: {description}
  
  {long description}
  

[ncfavier]

cc @chuangzhu @gou4shi1

[gou4shi1]

LGTM

[pjstadig]

I forgot to run ./format after adding the change to remove repetition. I've squashed everything down to a single commit.

I guess it needs a maintainer to approve the test run?