Given the following (config-tree) YAML:

```yaml
.default:
  foo: bar
config-1:
  baz: qux
config-2:
```

Intuitively, this is supposed to result in

```yaml
config-1:
  foo: bar
  baz: qux
config-2:
  foo: bar
```

This is not possible because `config-2` with only a following colon
results in a value of `null` instead of an empty dict.

Make that work as expected.

Part of the cleanups for datawarehouse#240



Signed-off-by: Michael Hofmann <mhofmann@redhat.com>