applets/digital-clock: use theme default font when fontFamily not set

When migrating from 5.25 config, the user may have previously set
boldText=true and fontFamily="". In this case, we need to use the
system font, and not the applet's (nonexistent) configured font.

This is required by plasma/plasma-desktop!1169

CCBUG: 459372


(cherry picked from commit 9bf9cc4f)