Don't error when decoding optional unparsed values

[natecook1000]

We're newly able to decode actual optional types due to allowing unparsed variable properties. "Normal" optional values are still wrapped in non-optional property wrappers, so ArgumentDecoder didn't need to handle optional values until now. Fixes #285.

Checklist

• I've added at least one test that validates that my change is working, if appropriate
• I've followed the code style of the rest of the project
• I've read the Contribution Guidelines
• I've updated the documentation if necessary

[natecook1000]

@swift-ci Please test