Support empty form versions when doing multi-version push to Central

[lognaturel]

Software versions

master (post https://github.com/getodk/briefcase/pull/872)

Problem description

https://github.com/getodk/briefcase/pull/872 introduces pushes of multi-version forms to Central. Currently, forms with blank version names are not supported.

Steps to reproduce the problem

1. Pull a form with submissions referencing multiple versions including a version with a blank name (no version attribute)
2. Push that form to Central

Expected behavior

A blank version should be treated the same as any other version.

Other information

There are a couple of issues. First, we need a way to represent empty form versions in the metadata json file. Then, we need a way to communicate the blank version name to Central. We currently use this endpoint to force a version on publish. It does not support forcing an empty version number. We could generate the submission locally and push it or perhaps consider changing the Central API.