Allow empty parameters.

[souphan-adsk]

Some implementations sign the payload with empty parameters.
As a result, since the library filters out any empty parameters, it couldn't be used to generate the expected signature.

This attempts to imitate the following statement from postman:
https://www.getpostman.com/docs/v6/postman/sending_api_requests/authorization

Note: Some implementations of OAuth 1.0 require empty parameters to be added to the signature.
You can select “Add empty parameters to signature” to add empty parameters.

[pboling]

This is great!