ensure_resource fails with undef parameter

Since puppet version 4.5.2 the puppet code below fails with a "Duplicate declaration", it worked in 4.5.1 and earlier releases. From the description of PUP-6385 I assume it was caused by the fix to that issue, although I haven't looked into what was actually changed.

puppet apply -e 'define foo($foo) { } ensure_resource("foo", "FOO", { foo => undef }) ensure_resource("foo", "FOO", { foo => undef })'