change: make nil_safe_casecmp judge compatible for nil-nil comparison

[mtasaka]

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 .

[ianks]

This makes sense to me and seems like the correct behavior. @samdoiron thoughts?

[ianks]

@mtasaka I think there is a linting failure in CI

[mtasaka]

@mtasaka I think there is a linting failure in CI

@ianks I've force-pushed my PR with indentation fixed. Would you recheck this?