Check for null type trees in dependency extraction

In some cases the dependency extraction may encounter a null `TypeTree`
(eg. arguments of macro annotations that are untyped). In such cases,
we simply ignore the node.

This solves all the crashes that I've been able to reproduce (https://github.com/sbt/sbt/issues/1655#issue-45323426, julianpeeters/avro-scala-macro-annotations#4, [Duhemm/macro-annotation](https://github.com/Duhemm/macro-annotation))

/cc @xeno-by @dragos