Read a non-attribute-set column as an empty attribute set

[kha333n]

Fixes #2687.

1.x only — 2.x already guards this, so no forward port.

A product whose attribute_data holds anything other than an attribute set
cannot be loaded: Product::find() throws, and so does every listing whose range
includes it.

The fix

get() decoded the column and iterated the result without checking it was
iterable. It now reads anything that is not an array as an empty attribute set,
which is what 2.x does.

is_array() rather than 2.x's ?: []: both catch null and 0, which are
the values that actually arise, and is_array() also catches a truthy scalar
that would otherwise reach the same foreach.

Rows reach this state through Lunar itself — a non-UTF-8 attribute value makes
set() store 0 (#2685), and 0 is one of the values get() cannot read.
Fixing that stops new ones, but the records already damaged stay unreadable, and
cannot be repaired through Lunar because Lunar cannot read them.

Touches the same file as #2686, which fixes the write side. No overlap in intent,
but whichever lands second wants a rebase.

Tests

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

• can load a record whose attribute data is not an attribute set — a dataset
  over the five shapes the column can hold: null, 0, "abc", 123 and
  malformed JSON. All five throw on 1.x.
• can still load attribute data that is an attribute set — passes before and
  after, and is the guard that the fix skips only non-attribute-sets.