Ensure xdg and xwayland string_prop() handlers deal with destroying views

Ensure xdg and xwayland string_prop() handlers deal with destroying views

When a view is destroyed (including override_redirect in the xwayland
case), the view_destroy() handler is called which checks for a currently
open A-Tab window switcher and causes an update there to remove the
destroying view from the list. Before view_destroy() is called, both
xwayland and xdg handlers reset the xdg_surface / xwayland_surface.

The window switcher update then creates a list of all windows which do
not have the 'skipWindowSwitcher' window rule property set. If there is
at least one 'matchOnce' window rule configured, this also tries to get
string properties of the destroying view which already had their
xdg_surface / xwayland_surface reset and thus run into an assert.

This patch fixes that so that the string_prop() handlers always return
an empty string in those cases rather than running into the assert.

For a more in-depth analyses and alternative solutions see the linked
issue.

Fixes #1082