Fix: Missing a nil check at StableDiffusionCLI

[1ucas]

I noticed that generating images with Swift using the example prompt caused the output image to always carry the suffix .str50.

Looking at the code I saw that image != "none" was never evaluating to false.

I believe there was a small confusion between Python's None and Swift's nil representation of null.

This super small PR fixes it.

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[1ucas]

I also would like to take the opportunity to say thanks to the maintainers and the community!
For us, iOS devs, these tools greatly reduce the initial gap of learning how to work with AI models.