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13845462 - [Tests] Adds a test to check if `image_embeds` None case is handled properly in `StableUnCLIPImg2ImgPipeline` (#2861)

[Tests] Adds a test to check if `image_embeds` None case is handled properly in `StableUnCLIPImg2ImgPipeline` (#2861) * improve stable unclip doc. * add: test to check if image_emebds None case is handled. * apply formatting/