waterWang opened a new pull request, #39734:
URL: https://github.com/apache/beam/pull/39734
### 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`:
```java
} 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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