Fix ObjectSpace.dump with super() callinfo

[jhawthorn]

super() uses 0 as mid for its callinfo, so we need to check for that to avoid a segfault when using dump_all.

Previously this would crash since #8571

require "objspace"
class Foo
  def foo
    super(bar: 123)
  end
end
ObjectSpace.dump_all(output: :string)

I also switched to using dump_append_string_value even though that's probably not necessary, it removes an assumption about what callinfo mids the parser will generate.

cc @peterzhu2118

[casperisfine]

I was just tracking that same crash 5 minutes ago ❤️