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

[gabry-lab]

What would you like to happen?

we can only use map<varchar, boolean> 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

"type": {
        "type": "map",
        "values": [
          "null",
          "boolean"
        ]
      }

Issue Priority

Priority: 3 (nice-to-have improvement)

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