set-variation to return None on tag not existing as per docs

[asibahi]

align code with docs

[RazrFalcon]

Make sure it doesn't break rustybuzz. Maybe just the docs are outdated.

[asibahi]

in rustybuzz Cargo.toml

[dependencies.ttf-parser]
git = "https://github.com/asibahi/ttf-parser.git"
rev = "52a98111ad146397de89cb2481ff5ec3ef562912"
# version = "0.25"
default-features = false
features = [
    "opentype-layout",
    "apple-layout",
    "variable-fonts",
    "glyph-names",
    "no-std-float",
]

All cargo test tests pass too.

(Aside: I came across this while porting ttf_parser to Zig)

[RazrFalcon]

Thanks. Hopefully someone would merge it soon.

I came across this while porting ttf_parser to Zig

Good luck 👍