shopify-scripts: Null pointer dereferences in ary_concat

@ssarong reported a null pointer dereference in MRuby: https://github.com/mruby/mruby/issues/3532 The issue was corrected in https://github.com/mruby/mruby/commit/d35fcf1ea2bfc3933b2f2543265874669feeee95...