fix: null should be preserved in relative navigations

fix: null should be preserved in relative navigations

The fix is a bit more complicated that I anticipated, I will come back
to this later on as the currently documented version works perfectly.

- the nullish params are removed before being passed to the matcher
- The encodeParam function transform null into ''
- The applyToParams also works with arrays but it makes no sense to
  allow null in array params

Ideally, I would make the matcher a bit more permissive so the encoding
is kept at the router level. I think the matcher sholud be responsible
for removing the nullish parameters but that also means the encode
function should leave nullish values untouched. We might need an
intermediate Type for this shape of Params, it gets a little bit tedious
in terms of types, so I would like to avoid adding more types.

Close #1893