1.13.1 - Fix comparisons between non-`nil` values and `nil`

What's Changed

Fix comparisons between non-nil values and nil by @gwynne in #138

The changes in #135 introduced a nasty bug where conditions of the form #if (value == nil) or #if (value != nil) would incorrectly treat nil as equal to any non-nil value. Unfortunately, there was no test for this case, so the problem was not caught. This fixes the issue and adds the missing test.

This patch was released by @gwynne

Full Changelog: 1.13.0...1.13.1