Relation.canonical getter throws when href contains no path

[Munter]

Munter/netlify-plugin-hashfiles#114 shows that if a relation with just a domain in its href is checked for being canonical with the Relation.canonical getter, we can throw on attempting to call .startsWith on null at https://github.com/assetgraph/assetgraph/blob/master/lib/relations/Relation.js#L278

We should add a check to ensure that both hrefObj and canonicalRootObj have non-null path properties

[Munter]

I can't really figure out how to recreate the setup that caused this, so I'm reluctant to make a change to the code when it won't be covered by a test