Add nil checks for Source types validation to be pointer receiver based (#2378)
Add nil checks for Source types validation to be pointer receiver based (#2378)

It's better to have nil checks inside the Validate method of the
type itself instead of the type that holds a field of that type
because this allows using the types in other contexts without
the types that hold a field.

Signed-off-by: Pierangelo Di Pilato <f5f1d2015efd0bb51da99f56609dc938930158a7@redhat.com>
knative/pkg,knative/pkg