Remove blank objectives

[davidsbailey]

Starts PLAT-1390.

• Step 1 (this PR): remove existing blank objectives, and prevent more from being added via levelbuilder UI
• Step 2 (make objective description required #43711): add validation preventing blank objectives from being added more generally

What i did in this step:

• Objective.all.reject(&:description).count returned 1 in development, levelbuilder and production
• used lesson edit page locally to remove the one problem objective, and added the resulting .script_json changes to this PR
• updated the lesson save path to never add blank objectives.
  • I couldn't provide a default value of '' when storing the description in the database, because that gets transformed to nil somewhere, possibly in serialized_properties.rb

Testing story

• new unit test
• manually verified that after entering a blank objective and saving the lesson edit page and then reloading it, the blank objective is gone

Deployment strategy

• merge step 1
• wait for it to reach levelbuilder and production
• check that levelbuilder and production do not contain blank objectives
• merge step 2