fix: Handle nil values in SDL render to prevent inspection errors

[dorian-marchal]

Inspecting a schema blueprint with pretty: true was crashing with an "Unexpected nil" error:

#Inspect.Error<
  got RuntimeError with message:

      """
      Unexpected nil
      """

  while inspecting:

      %{
        __struct__: Absinthe.Blueprint,
  ...

Changes

• Changed the render function to just return an empty string instead of raising.
• Added a test to make sure inspection doesn't crash anymore

Heads up

This fixes the crash, but I'm not totally sure this is the right approach. It's likely that nil values shouldn't be showing up here in the first place, and this may just cover a deeper issue.

Fixes #1185

[dorian-marchal]

This fix is probably invalid, opened an issue with more information instead: #1403