Add NavigationHelper check for valid step points using lineSlice

[danesfeder]

Closes #932

[Guardiola31337]

What about consolidating these two guard clauses 👇 (that return the same) extracting the conditional into a explaining variable?
Also what about creating stepIntersections after the guard clauses?

Source code structure
Variables and methods should be defined close to where they are used.
Local variables should be declared just above their first usage and should
have a small vertical scope.

Clean Code (Robert C. Martin)

That way we avoid allocating it if any of the guard clauses is executed.

[Guardiola31337]

Could we return Collections.emptyList() to make it more explicit? If so we can move define distancesToIntersections down, close to where it's used. BTW we should check if there's any implication on returning an immutable list in this case 👀 https://stackoverflow.com/questions/5552258/collections-emptylist-vs-new-instance#answer-5552287

[Guardiola31337]

Why are we creating stepIntersections here? We could check if intersections is empty (instead of stepIntersections.isEmpty()) and define it down (after the guard clause), close to where it's used. Am I missing something?

[Guardiola31337]