[Feature Request]: calcite SQL support map with a nonnull value

### What would you like to happen?

we can only use `map not null` in beam sql, the value of map can be null, however, when I debug a row of sql output, its avro schema is
```java
"type": {
"type": "map",
"values": [
"null",
"boolean"
]
}
```

### Issue Priority

Priority: 3 (nice-to-have improvement)

### Issue Components

- [ ] Component: Python SDK
- [X] Component: Java SDK
- [ ] Component: Go SDK
- [ ] Component: Typescript SDK
- [ ] Component: IO connector
- [ ] Component: Beam examples
- [ ] Component: Beam playground
- [ ] Component: Beam katas
- [ ] Component: Website
- [ ] Component: Spark Runner
- [ ] Component: Flink Runner
- [ ] Component: Samza Runner
- [ ] Component: Twister2 Runner
- [ ] Component: Hazelcast Jet Runner
- [ ] Component: Google Cloud Dataflow Runner

Start by tracing Beam SQL's handling of MAP types into the Avro schema shown in the issue. Check how a non-null value declaration is represented, then verify with a focused Java SDK test that the generated schema preserves that constraint. Done means MAP values are represented as non-null rather than as a nullable union.

Written by the indexing model from the issue text.