[Avro] Default value for no required fields to null

[patou]

When a field is not required, the avro schema has no default value.
Has described in an avro issues, the property default: null must be set.

If we don't do this in the avro file, when we generate java code from generated avro schema, the builder fail if the data isn't set in the avro data class.

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[wing328]

cc @sgadouar

[sgadouar]

i agree, must add "default": null id the field is nullable