applets/keyboardlayout: fix warning on startup

The applet sets the font point size to the applet height. While the applet is loading, its height is 0, which causes QFont to complain as it prefers its point sizes strictly greater than zero.

Instead, just set it to something reasonable if falsy - taking the Kirigami default point size here because it sounds reasonable; if the applet is displayed it'll have a height anyway so the actual value doesn't really matter.