Mark informative audit N/A when no tag loads.

[jburger424]

With the implementation of the new audits "Deprecated GPT API Usage" and "GPT Errors", pages without GPT (e.g. example.com) will score a 0 for Publisher Ads.

Before, the category score would be set to N/A on these sites, but because these are not set as N/A, the category score is being set according to the 2 applicable audits.

I'll file a bug in LH to discuss this, as it seems like the wrong default behavior for informative audits, but in the meantime it'd be good to fix the issue on our end.

+CC: @warrengm