deserialize encryption context should fail on malformed encryption context

If the client deserialization receives a malformed ciphertext that defines the AAD length as 0 and then also defines a AAD fields as 0, the deserialization logic SHOULD raise an error. It does not. Instead, it accidentally interprets it as an empty encryption context.

https://github.com/awslabs/aws-encryption-sdk-python/blob/master/src/aws_encryption_sdk/internal/formatting/encryption_context.py#L159-L164

Example malformed test vector:
```
b'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'
```