[api-extractor] `ExtractorConfig.prepare` fails if `configObject.docModel.apiJsonFilePath` is not provided

[vjpr]

This happens when programmatically building the config file.

There is an unhandled error when this value is not provided. It tries to write to a file with filename ''.

This should throw a nice error like some of the other settings do.

It would be nice if there was a default IConfigFile that could be used.

[octogonz]

@vjpr Can you provide a repro?