length-unit: Also fall back if gtk-xft-dpi is default value
The check introduced in a7738a4d only uses the fallback value if gtk-xft-dpi is zero, but by default (if a backend doesn't define it) it is -1.
This is required for the GTK Android backend as it currently suffers the same issues described in #809 (closed) (and I don't want to to provide a false gtk-xft-dpi via settings, because text will become comically large if I were to use the right value)