fix: read back the maintained-index selection a table was given (#4447)

`maintained_indexes` on a MemWAL table has three values: a named list,
`[]` for none, and `null` for every index the table has, including ones
created later. This PR makes every client read all three back as they
were set.

- **Rust remote client** (also used by the Python and Node SDKs) reads
`null` as `None`.
- **Java client** reads `null` as `null`, not as an empty list.
- **Docs** for the setter, the field and the getter describe the three
values the same way in Rust, Python, Node and Java. The generated JS
docs are regenerated to match.

## Testing

- `rust/lancedb`: 73 LSM tests pass, including one case per value. With
the decoder typed as a list, the `null` case fails.
- Java: two cases for `null` and `[]` on the read path, run by the Java
CI job.
- `cargo fmt`, `cargo clippy -D warnings` and `ruff format` are clean.

java/README.md

@@ -67,7 +67,7 @@ reader usually expects:
 
 | Value | Meaning |
 | --- | --- |
-| unset (null) | Maintain **every** index the MemWAL can, resolved on install |
+| unset (null) | Maintain **every** index the table has, including ones created later |
 | `Collections.emptyList()` | Maintain **none** |
 | `Arrays.asList("id_idx")` | Maintain exactly those |