fix: allow empty DynamoDB list and map in AttributeValueCoder

[waterWang]

Problem

AttributeValueCoder.encode cannot encode a DynamoDB attribute whose value is an empty list or an empty map — it throws CoderException("Unknown Type").

The L and M branches are guarded with size() > 0:

} else if (value.l() != null && value.l().size() > 0) {     // wrong: DynamoDB allows an empty L
} else if (value.m() != null && value.m().size() > 0) {     // wrong: DynamoDB allows an empty M

Fix

Remove the size() > 0 guard for L and M branches. The decode side already handles empty lists and maps correctly via ListCoder/MapCoder — only encode rejects them.

Blast radius

A single empty list nested anywhere inside an item makes the whole item unencodable, because MAP_ATTRIBUTE_CODER re-enters this coder for every child.

Fixes #39730

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