Do not add post form body to query params if form data is only set via event

[lukasbableck]

In a scenario where you have an empty form, so no inputs and no name attribute on the submitter, and you're setting the form data via the turbo:submit-start event (with event.detail.formSubmission.fetchRequest.body.append), you're actually adding the form data to the URL objects searchParams, as resource.searchParams is used for the body.
This causes the URL to contain all of the form data, which is especially problematic for larger amounts of data, as the webserver then might just close the connection because the URL is too long. In my case this happened after the URL reached about 7.5KB.
This PR fixes the issue, while also simplifying the logic a bit.

/cc @m-vo

[m-vo]

The fixed bug is really subtle as most of the time you won't realize that you're working with the reference of the actual browser URL in a turbo:submit-start event. 👍

Thanks for the fix @lukasbableck - removing the case for when the body length is 0, also makes the code much easier to understand.