Favor clipboard image when there's no surrounding text

[dae]

From time to time when pasting an image to a note, an error message occurs with message as in the topic name. The image pastes to the note, though.
Version of Anki: 24.04.1

We try the HTML content first, because the user may have selected both text and an image at the same time. But if we know there's only an image, using the image content on the clipboard would let us avoid a network request. Perhaps if there's an image on the clipboard, we can safely assume there's no surrounding text, even if there's also HTML in the clipboard?

Originally reported on https://forums.ankiweb.net/t/an-error-occured-while-opening-https-connection-pool/44459/2

[abdnh]

This can reintroduce some issues noted here: 174a2e7