Informative audits break category 'N/A' scoring

Informative audits with weight 0 are treated as applicable and will make the category score 0 when all other audits are marked not-applicable.

Context: https://github.com/googleads/publisher-ads-lighthouse-plugin/issues/261 I think that this change should be made regardless on our end, as we shouldn't be showing audits when not applicable.

As these are weight 0 and don't affect the scoring, I think they should be treated as not applicable when computing the displayed value.

I'd be happy to implement this myself.