Remove error from image loader builder

[rainest]

The image loader addon returns an error if given an empty string as an image. This is a relic of my old, bad design that expected this to occur because the input was an envvar.

Builders returning errors makes them annoying to work with, and callers should be responsible for not attempting to build no-op addons, not the addon. This case is ultimately handled later anyway, since Deploy() won't be able to load the non-existent image and will return an error (and addon deploy functions should return errors, unlike builders).

• Remove the error handling from the load image builder.
• Mark a breaking change in the changelog indicating that callers should instead check if they actually need to load an image before invoking this.