Ensure free response questions can be answered

[rtibbles]

Summary

• Fixes issue where free response questions with no set answers (i.e. all of them) would get set as having a float value in the QTI.
• Adds regression test and defaults to string value when no answers are given
• Update to check all candidate answers, so that users are free to input specified incorrect answers

References

Fixes the Studio aspect of learningequality/kolibri#13754

Reviewer guidance

Create a survey in your local Studio. Add a free response question.
Import it into Kolibri using code from this PR: learningequality/kolibri#13791

Ensure that you can enter text responses and submit.

Screenshot of a successfully submitted survey response in the report:

[marcellamaki]

code changes make sense, and cross testing this with corresponding Kolibri PR 13791 confirms that I can submit a text free-response answer