Fix duo authorization without preferred device

[yermalov-here]

Description

Fixes handling of the case when there is no preferred_device input in the duo login form.

Related Issue

#478

Motivation and Context

Fixes authorization for users with Duo hardware tokens.

How Has This Been Tested?

Added a test with a new fixture login-form that is based on the actual service response.
Tested the fixed case locally by running the tool.

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

[yermalov-here]

@epierce, could you please take a look at this change