xkbcomp/keymap: use default interpret in ApplyInterpsToKey

xkbcomp/keymap: use default interpret in ApplyInterpsToKey

This makes the code easier to follow and does more explicitly what the
xkblib spec says:
     If no matching symbol interpretation is found, the server uses a
     default interpretation where:
        sym =	0
        flags =	XkbSI_AutoRepeat
        match =	XkbSI_AnyOfOrNone
        mods =	0
        virtual_mod =	XkbNoModifier
        act =	SA_NoAction

If a level doesn't have any keysyms, we don't apply anything to it.

Signed-off-by: Ran Benita <ran234@gmail.com>