NULL values are implicitly serialized

[blag]

Converting null values is difficult because the ruamel package doesn't explicitly serialize None to null - it simply leaves the value blank.

Observe:

input:
  - foo
  - bar
  - baz: null

Which reserializes to:

input:
  - foo
  - bar
  - baz:  # implicit null

Luckily empty values are deserialized to None anyway, so this doesn't break anything, but it does impact readability.

This is reported in ruamel issue #169.