Empty "test" elements in "program"

[rbeezer]

Lead to emitting ==== and a blank line. Which the Runestone back-end rejects.

Normalize the content and test for an empty string, and don't create the equals signs.

[rbeezer]

Implemented at 87ef2dc0f83ca263ce1fe28b9beac2265c9ceafc and subsequent.

For @bnmnetp:

• Should I make a similar "fix" for tests on a horizontal Parsons problem?
• Activity 5.3.1 of the sample book now has an empty tests in it. You could check behavior on next automated build?