Empty value on solid-form-image should not return null

Core 0.17.

With a solid-form, using a solid-form-image*, having a blank field should not be rewritten as null when saving.

Example jsbin: https://jsbin.com/saguqaziju/1/edit?html,output

Notice that here, the data-src does not exist, because of authentication I wasn't able to make a proper jsbin.
It's the exact same behaviour with an existing resource, containing a someimage: "" instead of someimage: null value.

@balessan I'd like to have this one fixed before the release for SFIH, it's rather disturbing when a user save a community without a picture. And it save "null" as a string as a picture when the user save the form two times.

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Before saving:

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After saving: