Inconsistent 'empty' return values

[wb9688]

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.