Don't emit warning when encountering link with none

[Its-Just-Nans]

Potential fix for #993

[LaurenzV]

Two things:

1. I think this doesn't only apply to markers but all other types as well (for example clip paths, so we should probably not restrict the check for markers.
2. I think it would be better to do the check earlier, like so:

    /// Returns an attribute value.
    pub fn attribute<T: FromValue<'a, 'input>>(&self, aid: AId) -> Option<T> {
        let value = self
            .attributes()
            .iter()
            .find(|a| a.name == aid)
            .map(|a| a.value.as_str())?;
        
        if value == "none" {
            return None;
        }
        
        match T::parse(*self, aid, value) {
            Some(v) => Some(v),
            None => {
                // TODO: show position in XML
                log::warn!("Failed to parse {} value: '{}'.", aid, value);
                None
            }
        }
    }

[Its-Just-Nans]

Make sense, fixed

[Its-Just-Nans]

The simple if value == "none" is not great looking from the test

Here is another fix

[LaurenzV]

There probably is some better fix for this, but I also can't think of anything off the top of my head that wouldn't require larger changes, so seems good to me. Thanks!

[sww1235]

Thanks for fixing this so quickly!