nested_exception: fix backtrace parsing

While working on airbrake/airbrake#479 I
stumbled upon a bug in Airbrake Ruby where the default filter couldn't
process error payload because `:backtrace` was `nil`. Returning an
empty array in case the real backtrace is missing seems to be a more
secure approach.

lib/airbrake-ruby/nested_exception.rb

@@ -35,7 +35,7 @@ def unwind_exceptions
     end
 
     def parse_backtrace(exception)
-      return if exception.backtrace.nil? || exception.backtrace.none?
+      return [] if exception.backtrace.nil? || exception.backtrace.none?
       Backtrace.parse(exception)
     end
   end

spec/nested_exception.rb → spec/nested_exception_spec.rb

@@ -68,8 +68,8 @@
           exceptions = nested_exception.as_json
 
           expect(exceptions.size).to eq(2)
-          expect(exceptions[0][:backtrace]).to be_nil
-          expect(exceptions[1][:backtrace]).to be_nil
+          expect(exceptions[0][:backtrace]).to be_empty
+          expect(exceptions[1][:backtrace]).to be_empty
         end
       end
     end