When a new review was posted without body_top text and no comment, we

fail to set the body variable, causing a crash.
While the crash is harmless, it does cause spurious issues in the logs.
This change fixes this, and improves the overall behavior in two ways:
1) If the body_top is "Ship It!", we now fall back to the first comment

   (if available), rather than an empty string for the text.
2) If a comment is not available, it will now fall back to body_bottom,

   which wasn't being considered before.

Unit tests pass. The new tests previously exhibited the behavior reported

by a customer.