Make rubric prompt display and instructions optional

TIM-410: if users want rich prompts, they can do that with an HTML
Components above the ORA2 question, but doing this makes the prompt box
look bad. This adds a check so that the prompt box will not display if
the <prompt> XML tag is missing is empty or whitespace-only. Also
extends the serializer to serialize missing prompts distinctly from
empty prompts.

Includes tests for each of these conditions.