fix: #3628721 Keep region content rendering when core renders regions via #theme

Core issue #3178202 renders page regions with '#theme' => 'region', so core's region preprocess provides "content" as an array of block render arrays and #children is empty when the template runs. Stop overwriting "content" with #children, only add the help alert when the help region renders something, and test the rendered output for emptiness in the region templates. No change on core without #3178202.

Closes #3628721

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