Issue/structure builder throws exception 1001707

Calling `List.head` is in general a bad idea, so that alone should be good
motivation for not doing it. Furthermore, as ticket #1001707 demonstrates, the
call to `head` can throw an exception in the `ScalaStructureBuilder`.

The most likely reason why that could happen is because the source contains
type errors, and hence not all expected information are available in some of
the inspected symbols. For instance, a class is expected to always have at
least one superclass, however if there are type errors it is possible for a
class symbol to have no superclasses, and hence accessing the `head` of an
empty `List` could result in a `NoSuchElementException` being thrown.

This fix comes with no tests because the ticket doesn't provide a reproducible
test case.

Fix #1001707

backport to _release/3.0.x_