Inspector should allow explicit None for delegate_map (#4136)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/4136

The SDK inspector should allow parsing delegates that have a `None`
for their delegate_map. This is explicitly None, and not unspecified, which
is why the default of an empty dict didn't work.

Use `or {}` to instead detect None.

Reviewed By: Olivia-liu

Differential Revision: D59298227

fbshipit-source-id: 8503d680acb4c393459ee2956bfd7b9f4725803c