Iterable return types should return empty array by default

[Slamdunk]

Without this fix, mocked method with traversable|Generator|iterable return types return an array with one null element:

https://3v4l.org/mR8kdU

1) ./tests/end-to-end/mock-objects/generator/return_type_declarations_generator_empty_by_default.phpt
Failed asserting that string matches format description.
--- Expected
+++ Actual
@@ @@
-array(0) {
+array(1) {
+  [0]=>
+  NULL
 }
-array(0) {
+array(1) {
+  [0]=>
+  NULL
 }
-array(0) {
+array(1) {
+  [0]=>
+  NULL
 }