Fix creating from non-stashbox sources in Scene Tagger

[DingDongSoLong4]

This fixes two bugs in #3912 that came up in Discord related to using non-stashbox scrapers in the Scene Tagger:

• When you create a new performer, the created performer replaces all performers that were previously about to be associated with a scene.
• And creating a studio does not work at all.

I also found a different bug while trying to reproduce the above issues, where the "Create Studio" dialog still shows the (blank) scraped image and its checkbox even if the scraper doesn't return an image. The simple fix is just to not render the image if the path is missing.