Validate that credential statements are not empty

[orhoj]

Purpose

Fix an issue where the proof prompt was just a blank screen. This was due to receving a request with no statements, which is not a valid request.

Changes

• Validate that statements is non-empty.

Checklist

• My code follows the style of this project.
• The code compiles without warnings.
• I have performed a self-review of the changes.
• I have documented my code, in particular the intent of the
  hard-to-understand areas.
• (If necessary) I have updated the CHANGELOG.

[orhoj]

I update the CHANGELOG for this in #361.