Fix BlockViewBuilder to handle NULL and []

Problem/Motivation

Follow up from 📌 Improve debugability of block plugins returning NULL in Layout Builder Needs work

From lauriii

It looks like \Drupal\block\BlockViewBuilder::preRender also explicitly adds support for NULL type, even though it's not documented to be supported. 😬 As the error suggests, there are already type hints for ensuring it is an array, meaning that it's fine to add the exception as a DX improvement. However, we probably should allow NULL as the empty value as well, since it's supported by \Drupal\block\BlockViewBuilder::preRender.

From @tim.plunkett

Good news: returning [] and NULL have exactly the same effect. We can file a follow-up to fix BlockViewBuilder which has been wrong this whole time.

Steps to reproduce

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Proposed resolution

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Remaining tasks

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User interface changes

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Introduced terminology

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API changes

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Data model changes

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Release notes snippet

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