Fix nullable schema generation for non-required dictionary fields

[RobertIndie]

Motivation

Currently, fields of type Dictionary are generated as non-nullable Avro map schemas, for example:

{
  "name": "MyMap",
  "type": {
    "type": "map",
    "values": "string"
  },
  "default": {}
}

This causes serialization to fail when an optional dictionary field is unset, because the generated schema does not allow null values:

Unhandled exception. Avro.AvroException: Map is null - use a union for nullable types in field MyMap
 ---> Avro.AvroTypeException: Map is null - use a union for nullable types

Modifications

• Make non-[Required] Dictionary<TKey, TValue> fields generate nullable Avro union schemas.
• Keep [Required] dictionary fields non-nullable.

Non-required dictionary fields now generate:

"type": [
  { "type": "map", "values": "string" },
  "null"
],
"default": {}

Required dictionary fields still generate:

"type": { "type": "map", "values": "string" },
"default": {}

[RobertIndie]

@eaba Could you help review this PR? Thanks!