Properly handle YAML null macros for default_value_if
Properly handle YAML null macros for default_value_if

The macro `yaml_opt_str` is only used in `yaml_tuple3`, which again is
only used for `default_value_if`. Unfortunately, the current test
doesn't make sense, as a v.is_null() indicates a Yaml::Null, on wich
`as_str()` always returns `None`. Instead, the condition should be
negated, as the documentation of `default_value_if` hints:

> **NOTE:** If using YAML the values should be laid out as
> follows (`None` can be represented as `null` in YAML)

The case `$v.is_null()` should therefore lead to `None`, whereas all
other cases should be interpreted `as_str()`.

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