feat(content): allow multipart requests without files.

[laipz8200]

Summary

related #3396.

Currently, a multipart request can only be sent when the files parameter is non-empty. This restriction limits cases where users might want to send data using multipart mode without attaching any files, which is possible in tools like Postman.

Change: Instead of requiring files to be non-empty, we could determine the need for a multipart request based on whether files is None rather than its emptiness. This would allow users to send multipart requests without attaching files if needed.

I think there is no significant documentation update required, so I have not made any changes. However, please let me know if any updates are needed.

Checklist

• I understand that this PR may be closed in case there was no previous discussion. (This doesn't apply to typos!)
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• I've updated the documentation accordingly.

[tomchristie]

Thanks. I've updated the tests to avoid private imports.

Do we think this is an improved behaviour? It does seem nice that an explicit files={} will enforce multipart. (On the other hand, if data and files are determined dynamically as in the cli we need to explicitly switch to None)

[laipz8200]

Thanks for the update!

My original plan was to make it easier to use multipart without having to carry files, which wasn’t as straightforward before.

I don’t use the cli much, so if this causes any issues, just let me know.

[sylann]

I don't think this change is correct. It seems to go against the spirit of your main change in the httpx/_content.py file.
It seems to prevent the fix from working when using the cli.
Although I am not familiar with this usage and I didn't look too much into it, so I don't know if it's even possible to do this through the cli.

[laipz8200]

Thank you for the review, I'll take a look at the CLI.