NPE if reading record without a amzn-ddb-map-desc

This line throws a null pointer exception if you attempt to decrypt a record where the amzn-ddb-map-desc attribute has been manually removed: https://github.com/aws/aws-dynamodb-encryption-java/blob/master/sdk1/src/main/java/com/amazonaws/services/dynamodbv2/datamodeling/encryption/providers/DirectKmsMaterialProvider.java#L126

We should check this earlier and throw a more useful exception.

从 sdk1/src/main/java/com/amazonaws/services/dynamodbv2/datamodeling/encryption/providers/DirectKmsMaterialProvider.java 的第 126 行开始，在手动移除 amzn-ddb-map-desc 后复现解密过程。跟踪缺失属性首次被使用的位置。完成标准是缺失属性产生有用的 exception，而不是 NullPointerException。

由索引模型根据 Issue 内容生成。