[YAML] Enable to use empty strings as default values in property definitions

[miwurster]

Currently, it is not possible in Winery to define an empty string as the default value for a property definition (as well as other definitions, e.g., attributes, capabilities, requirements). This requires extending the parser/serializer to properly handle such a scenario.

Source: https://github.com/radon-h2020/radon-gmt/issues/50