silence Variant#in_stock? warning for correct usage

See comment here: https://github.com/spree/spree/commit/a7e6bc1#diff-f61dbf6358707ede7a8f24c85692baa5R147

It looks like it's still OK to call in_stock? without any arguments. If that's true then let's not print a deprecation notice when the method is called without arguments.