glyph_variation_data returns Err for glyphs with no variation

[simoncozens]

font.gvar()?.glyph_variation_data(GlyphId::new(0)) for a font where .notdef does not vary returns "Error: An offset was out of bounds".

This is surprising, because nothing actually went wrong, and I didn't even provide any offsets, so this looks to the user like a bug in the library. If the return type was Result<Option<...>> or Option<Result<...>>,instead of Result, then the lack of variation could be returned more clearly.