* Skips validation where there is no attribute definition, but keeps the attribute value. Previously, the attribute would be omitted from the attributes passed to `render_callback`. Notably, this resolves an issue where `render_callback` cannot receive a block's `align` and `customClassName` attribute values, since these are defined as a client-side filter.
* Validates `null` as a proper value in its own right. Previously, a client implementation of a block could track `{"attribute":null}` as an explicitly empty value, and the server would wrongly initiate defaulting behavior. The new behavior will now only populate a default value if the attribute is not defined at all, including when unset in its being invalid per the attribute schema.
Props aduth, noisysocks, youknowriad.
See #45145 for the patch, #45098 for the original ticket.
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These are the foundational classes allowing blocks to be registered and used throughout WordPress.
This commit also includes the `has_block()` and `has_blocks()` functions, which are required for unit testing these classes.
Props adamsilverstein, danielbachhuber, desrosj.
See #45097, #45109.
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