Using PyYAML, if I read in a file with blank values in a dict:

```
test_str = '''
attrs:
  first:
  second: value2
'''

```

This returns `None` for the key `first`:

```
>>> data = yaml.load(test_str)
>>> data
{'attrs': {'second': 'value2', 'first': None}}

```

But when writing, the `None` value is replaced with `null`:

```
>>> print(yaml.dump(data, default_flow_style=False))
attrs:
  first: null
  second: value2

```

Is there a way to format the dump output to print a blank scalar rather than `null`?