`gtk_level_bar_get_offset_value()` returns `gboolean` but it is not a predicate.
The return value represents whether the offset named `name` exists or not.

Ref.: https://developer.gnome.org/gtk3/stable/GtkLevelBar.html#gtk-level-bar-get-offset-value

We should define it as `level_bar.offset_value(name) # => Float or nil`.