[Themes] Simplify ThemeFileSystem.files default values

[jamesmengo]

ThemeFileSystem.files have an attachment, or a value

Currently, we save '' instead of undefined. For example, images would have:
{attachment: '<base64_string>, value: ''}

We should either update the type signature to reflect this, or update the default value to set undefined

Suggestions

• We can consider consistency with the API or simplicity of CLI code when making this decision.
• Personally, I think modelling how the API views this is a good first pass approach. We should set the empty value to undefined and verify any fallback logic of || or ?? doesn't break. (This could break if we are expecting the '' value in the LHS of || to be considered truthy.

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          This should stay `||` imo. This will reflect the behaviour in [theme-fs](https://github.com/Shopify/cli/blob/55307bb53694f67022862d84de5f5e85744feec7/packages/theme/src/cli/utilities/theme-fs.ts#L60-L75), which saves `''` instead of `undefined` 

Now that I think about it, we could probably consider changing that across the codebase (should be done separately if so)

Originally posted by @jamesmengo in #4732 (comment)