Fail loudly when attribute data cannot be encoded

[kha333n]

Fixes #2685.

The same code is on 2.x at Casts/AsAttributeData.php, so this wants a forward
port.

One byte from a Latin-1 product feed replaces the whole of attribute_data with
0, taking every attribute on the record with it.

The fix

json_encode() reports failure by returning false, and that false was bound
straight into the update. Encoding now uses JSON_THROW_ON_ERROR, so a value
that cannot be represented fails the write instead of overwriting what was
already there.

The cast runs on assignment, so the bad value never reaches the database — which
also means this is not a new failure mode for callers: on 1.x the save already
threw, just after writing.

Nothing else changes, and get() is untouched.

Tests

tests/core/Unit/Base/Casts/AsAttributeDataTest.php:

• can not overwrite attribute data with an unencodable value — a product with
  a name, then a write carrying a single 0xE4 byte. Asserts the write throws
  and that the stored column and the readable name are exactly what they were.
  On 1.x the column is 0 afterwards and the product can no longer be loaded.
• can store attribute data containing multibyte characters — accented text,
  Japanese and an emoji all still round-trip. Passes before and after; it is the
  guard that the fix rejects unencodable bytes rather than non-ASCII ones.