Fix warning on Courses CPT "Level" column if the level found doesn't have a name (probably deleted)

[kimcoleman]

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Changes proposed in this Pull Request:

There was a warning shown for levels without a "name" (probably because they were deleted) on the All Courses CPT screen in the WP Admin. This fix checks that there is an existing level name, then adds to the array.

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Changelog entry

• BUG FIX: Fixed warning in Levels column of the All Courses screen in WP admin.

Enter a summary of all changes on this Pull Request. This will appear in the changelog if accepted.