[Refactor] save_model_card function in text_to_image examples

[tolgacangoz]

This pull request refactors the save_model_card function to handle the parameters of the images and repo_folder. It ensures that the function can handle both cases when images are provided and when not. len(images) was improper for the images' default None value. Also, None was improper as a default for os.path.join in terms of repo_folder.
@sayakpaul

[HuggingFaceDocBuilderDev]

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[sayakpaul]

I have left some comments. I don't think these changes are really necessary.

[sayakpaul]

The images none check is fine. Let's keep it.

[sayakpaul]

Thanks for beating it with me!

[tolgacangoz]

Thanks for reviewing and giving feedback!