in lib/boleite/src/boleite/backend/glfw_surface.cr:12: use `null?` instead of `nil?` on pointer types.

The semantic of `nil?` changed in the last version of the language
to mean `is_a?(Nil)`. `Pointer#nil?` meant "is it a null pointer?"
so using `nil?` is probably not what you mean here. If it is,
you can use `is_a?(Nil)` instead and in the next version of
the language revert it to `nil?`.

        unless @surface.nil?