Optional fields with null values should be counted as valid

[xrodo912]

Expected behaviour

Optional fields with a null value should be counted as valid if the structure matches.

Actual behaviour

Optional fields with a null value are counted as invalid.

Verify a yaml with this field/value:
middle name: null
Against this structure:
'middle name?': 'string'.

Versions and environment

• Operating system: Windows 10
• Node.js: 8.2.1
• npm: 5.3
• yaml-validator: 1.0.0

Thank you and have some 🍇.

[paazmaya]

Thank you for the issue, by following the template.

I will try to create a test case for this first...

[paazmaya]

@xrodo912 In case you have already some example files that could be used for testing, mind creating a PR?