Better handle cases when the xkb keymap fails to be created

Summary:
If the keymap cannot be created a few pointers in Xkb are null.
We should make sure to not call any xkbcommon functions on those
null pointers and instead use proper fallbacks.

This change introduces fixes for a few usages, but it's not unlikely
that there are more cases.

BUG: 381210
FIXED-IN: 5.10.3

Test Plan:
Autotest added for the condition of the bug, which does
not crash any more. Just starting the test found a few more crash
cases.

Reviewers: KWin, Plasma

Subscribers: plasma-devel, kwin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D6260