Type error prepared tag element with empty content.

This has no test coverage because (1) we don't have any empty tags in XBBCodeFIlterTest, and (2) even if we did, prepared tag elements are only used by plugin processors, the only plugin processor used in testing is Drupal\xbbcode_test_plugin\XBBCodeTestPlugin, and this plugin's ::prepare() method returns {prepared:{$content}}, which is never an empty string.

Opened MR with test-only. This should fail.

Added the fix. This should pass.

Perfect.

Automatically closed - issue fixed for 2 weeks with no activity.