null entry shoudl not be written as null, instead it should be written has blank

[Danny Dulai]

http://www.yaml.org/spec/1.2/spec.html#id2786563

states that blank values are 'null' -- so when doing json -> yaml, it should not be written as null:

% echo '{ "o": { "foo" : null } }' | json2yaml  -
o:
  foo: null

it should be:

% echo '{ "o": { "foo" : null } }' | json2yaml  -
o:
  foo: