Added support for optional values in arrays.

[AmerigoM]

Description

Added support for optional values in arrays.

Closes #22.

Checklist

Ensure that your pull request has followed all the steps below:

• [x] Code compilation.
• [x] All tests passing.
• [x] No new SwiftLint issues.
• [x] Added new unit tests, if applicable.
• [x] Extended the documentation (including README), if applicable.
• [x] Updated version in CBORCoding.podspec following semver guidelines.
• [x] Ran workflowtests.sh and passed.
• [x] Added myself to the CONTRIBUTORS file.