Fix empty image fields saving null or undefined.

[tech4him1]

- Summary

Closes #828.

Does anyone think we need a more through fix, i.e. returning an empty string instead of null or undefined?

- Test plan

- Description for the changelog

- A picture of a cute animal (not mandatory but encouraged)

[verythorough]

I think an empty string is great.

I suppose we could think about popping up a confirmation about being sure you actually want screen readers to completely skip this image, but that would get really annoying if you're using a lot of decorative images on a page. I think having the alt field is enough to nudge people in the right direction, and we don't need to teach them accessibility good practices beyond that. :)

[tech4him1]

I was wondering if we should send an empty string to all plugins, instead of undefined (vs just changing this for the image plugin). That could be a breaking change, though, so I'm not too keen on it.

[verythorough]

Nice catch on the spelling error!

I wonder if we need the default empty string for the src. Kinda seems like if that field should be required for the block, or if it's empty, the whole tag goes away.
Failing that, we might consider using "no image selected" as the default for the src, instead of an empty string.

[erquhart]

@verythorough I agree this could use more thought moving forward. As it stands it's an improvement, so merging for now.