Ruby returns 0 (not nil) for nil <=> nil, i.e. nil and nil are judged
as equal for comparison, and so returns nil_safe_compare .
ref: https://github.com/Shopify/liquid/pull/1476
To make the behavior of nil_safe_casecmp consistent with
nil_safe_compare , change nil_safe_casecmp so that comparison between
nil <=> nil return 0 (equal).
Also change testsuite to reflect this change.
Fixes#1759 .
