Resolves #10041
Resolves #10047

A few inconsistencies have remained in the grammar specification
concerning freeform blocks and blocks without attributes in the
block delimiters. Freeform blocks were returned without block
names and blocks without attributes returned `null` instead of
an empty set of attributes.

Further, the default parser implementation (from #8083) was
returning an array of block objects instead of an array of
generic arrays. This resulted in mismatches in PHP of accessing
properties with `$block[ 'attrs' ]` syntax vs `$block->attrs`
syntax.

In this patch I've updatd the specification to remove all of
the type ambiguity and have updated the default parser to match
it. After this patch every block should be accessible as a normal
array in PHP and have all properties: `blockName`, `attrs`,
`innerBlocks`, and `innerHTML`. If no attributes are specified
then `attrs` will be an empty set (in JavaScript `{}` and in
PHP `array()`).