Do not hyperlink text when LinkResolver returns null

Treat the link as broken if the LinkResolver returns null. This allows the application developer to decide when a link is broken (which may depend on context, as well as on `$link->isBroken()`), and log an error if they like.

FakeLinkResolver has been updated to return null when `$link->isBroken()` is true.

Behaviour before this patch was to render `<a href="">link text</a>`.

This fixes issue #21