Disallow contentless Ponder answers

Summary:
Any answer without non whitespace characters results in an
error.

Test Plan:

• Submitted an empty answer, was told off
• Submitted an answer full of whitespace, didn't like that either
• Submitted a real answer, accepted

Reviewers: pieter, epriestley, vrana

Reviewed By: epriestley

CC: aran, Korvin, davidreuss

Maniphest Tasks: T1644

Differential Revision: https://secure.phabricator.com/D3492