Allow import of run configuration folderName from Gradle / External Project Model

[shartte]

This enables JetBrains/gradle-idea-ext-plugin#87 to be solved, by importing the folderName model property for run configurations.

Empty strings are converted to null here, since the IDE expects null for "no folder" and doesn't react nicely to empty strings being set.

Tested using an adapted idea-ext to work (these run configs were imported from Gradle, including the folders):

No tests were adapted since I couldn't find any tests that actually tested this external model import.

---

Example on how to make use of this with idea-ext, without having to build a custom plugin:

var appRun = new Application("name", project) {
    @SuppressWarnings("unchecked")
    @Override
    public Map<String, ?> toMap() {
        var result = (Map<String, Object>) super.toMap();
        result.put("folderName", "RUN SUBFOLDER");
        return result;
    }
};
runConfigurations.add(appRun); // add as normal to the container

[shartte]

This might of interest to you @nskvortsov, if you're still maintaining idea-ext