Lesson Plans - make experience level field rendering conditional

[kathrynwp]

Issue

Now that we've removed the "Any" option in the Experience level taxonomy – as discussed in #2676 – there is a blank space where that level had been used previously.

See example at: https://learn.wordpress.org/lesson-plan/webp-images-in-wordpress/

Enhancement

Let's only display the experience level if an experience level has been assigned to a lesson plan.