Return undef from endpoints when there is not content in the response

Issue: https://rt.cpan.org/Ticket/Display.html?id=91368
Croak when a response don't have a Content-Type specified
v0.004

  [Fixes]

  - [RT-91368] Some resource responses contain no content but specify
    a Content-Type. Those were being considered errors. Now they
    simply return undef.

  - When a resource response contains a content but doesn't specify a
    Content-Type it's an error. Previously it was returning undef.