Only postprocess Lesson content if we have translations

[Hamms]

Otherwise, any language that hasn't translated any Lesson content will fail because result[:lessons] is expected to be nil in that case.

I recommend reviewing with whitespace changes disabled: https://github.com/code-dot-org/code-dot-org/pull/41374/files?w=1

PR Checklist:

• Tests provide adequate coverage
• Privacy and Security impacts have been assessed
• Code is well-commented
• New features are translatable or updates will not break translations
• Relevant documentation has been added or updated
• User impact is well-understood and desirable
• Pull Request is labeled appropriately
• Follow-up work items (including potential tech debt) are tracked and linked