[DialogShadows] Use 0 offset for disabled borders on Wayland

Summary:
On Wayland, DialogShadows doesn't attach empty tiles for disabled
borders. Yet, it sets offset of 1 for them. This results in visual
artifacts like this

KWin thinks that there is 1px tall shadow in the bottom.

In general we don't need to pass empty shadow tiles for disabled
borders. Thus, that's fine to set 0 offset for them.

Test Plan:

• Started kwin_wayland:
  
  dbus-launch kwin_wayland --windowed --xwayland

• Launched latte-dock and kate:
  
  unset DISPLAY QT_QPA_PLATFORM=wayland latte-dock & QT_QPA_PLATFORM=wayland kate

• Opened calendar popup:

Reviewers: davidedmundson

Reviewed By: davidedmundson

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D14946