[ActiveStorage] Prevent AS::Preview#processed to generate a variant for an empty transformation

[chaadow]

Followup #50044 cc @jonathanhefner

Motivation / Background

if an empty hash is passed to a preview call (blob.preview({})) We go through the original preview instead of regenerating a variation based on the original preview image which would result in a performance penalty.

Detail

Calls to #url or #key would now use the original blob.preview_image instead of generating a variant that has an empty transformation.

==> Resulting in saving both a MiniMagic/vips call. + a network/IO call to the storage service

Additional information

Checklist

Before submitting the PR make sure the following are checked:

• This Pull Request is related to one change. Changes that are unrelated should be opened in separate PRs.
• Commit message has a detailed description of what changed and why. If this PR fixes a related issue include it in the commit message. Ex: [Fix #issue-number]
• Tests are added or updated if you fix a bug or add a feature.
• CHANGELOG files are updated for the changed libraries if there is a behavior change or additional feature. Minor bug fixes and documentation changes should not be included.

[chaadow]

I've squashed and rebased with main.

[jonathanhefner]

I see that you recently pushed many documentation changes. Would you mind splitting those out into separate PRs unless they are directly related to this change? It makes things easier to review. Also, keep in mind that this is an optimization for a corner case, so I don't think we need to extensively document it (if at all).

[chaadow]

Would you mind splitting those out into separate PRs unless they are directly related to this change?

@jonathanhefner I changed all of processed, url, key and download, and they're publicly documented API, I thought I would update the documentation to reflect the new reality of these methods. Even though it's an "edge case", I thought documentation should elicit all possible cases.

Moreover, contextually speaking, since it's a followup from our past Pull request that we co-authored, in which we changed #processed but we forgot to update the documentation, (and I also re-changed this method again), I thought I might update to further help the readers with multiple examples that could potentially help them by adding more context.

I will revert and create another PR.

Thanks again for the prompt and exhaustive review!

[chaadow]

@jonathanhefner I've applied the last changes, squashed the commits, and rebased with main.

[jonathanhefner]

Thank you, @chaadow! ✔️

[nicowenterodt]

Thanks from me as well. Very useful 👏 @chaadow @jonathanhefner

[zzak]

@chaadow I've stumbled upon this now, why is this here? If I remove it the tests still pass for me.