getNullableGenericDefaultValue API should fail Map type with null as default value.

[guawang]

Currently getNullableGenericDefaultValue return a hashmap object for Map type with null as default value. For example:
{
"name": "fieldName",
"type": {"type":"map", "values":"string" },
"doc" : "test.",
"default" : null
}
For this field, the getNullableGenericDefaultValue api returns a HashMap object which shouldn't.

avro-util/helper/helper-common/src/main/java/com/linkedin/avroutil1/compatibility/SchemaValidator.java

Line 157 in b6769c3

}

in this place it should also validate if the defaultValue as Map object is empty or not