Unable to store empty strings as values

This happens because AWS refuses to encrypt empty payloads:

```
λ confcrypt aws add -n FOO -v "" -t String -p -k .econf

confcrypt: ServiceError (ServiceError' {
_serviceAbbrev = Abbrev "KMS",
_serviceStatus = Status {statusCode = 400, statusMessage = "Bad Request"},
_serviceHeaders = [("x-amzn-RequestId","e68c2e4a-791c-4920-97e2-f4778afa963a"),("Cache-Control","no-cache, no-store, must-revalidate, private"),("Expires","0"),("Pragma","no-cache"),("Date","Tue, 19 Jan 2021 17:11:04 GMT"),("Content-Type","application/x-amz-json-1.1"),("Content-Length","175")],
_serviceCode = ErrorCode "Validation",
_serviceMessage = Just (ErrorMessage "1 validation error detected: Value at 'plaintext' failed to satisfy constraint: Member must have length greater than or equal to 1"),
_serviceRequestId = Just (RequestId "e68c2e4a-791c-4920-97e2-f4778afa963a")
})
```

Start from the `confcrypt aws add` entry point and trace how the `-v ""` value is passed to AWS KMS. Reproduce the command shown in the issue, then verify that an empty String value can be stored successfully without the KMS validation error.

Written by the indexing model from the issue text.