api: fix type error when contract state not found

[meevee98]

When contract state is not found the results in response returns None, because of this interactions using find_states result fail with TypeError (since None cannot be iterated)

[coveralls]

coverage: 80.514% (+0.02%) from 80.493%
when pulling d73374b on type-error-empty-storage
into e16f79c on master.

[ixje]

When contract state is not found the results in response returns None, because of this interactions using find_states result fail with TypeError (since None cannot be iterated)

This is an issue with neo-go because the C# client returns an empty list.
Note that the proposed solution will still throw a KeyError. We can work around it using response.get("results", ()) although I'd rather see neo-go fix it.

[ixje]

actually, you're solution is correct. Ignore that part

[ixje]

Thanks!