Inconsistent 'empty' return values

NewPipeExtractor has inconsistent return values in some places for when there is e.g. no HLS URL, i.e:

media.ccc.de

    @Nonnull
    @Override
    public String getHlsUrl() throws ParsingException {
        return null;
    }

PeerTube

    @Override
    public String getHlsUrl() throws ParsingException {
        return "";
    }

SoundCloud

    @Nonnull
    @Override
    public String getHlsUrl() throws ParsingException {
        return "";
    }

HLS URLs aren't the only case where that happens.

Start by comparing the getHlsUrl implementations in the media.ccc.de, PeerTube, and SoundCloud extractors, then inspect other extractor methods with similar empty-value behavior. Establish a consistent convention for absent values and apply it across the affected cases; done means the inconsistent null and empty-string returns are resolved consistently.