Allow creating an empty plugin

@Xpirix thanks for the feedback. Could we then allow to create an empty plugin (i.e. without a zip file)? It would be invalid (without any version) until it's pushed?

_Originally posted by @3nids in https://github.com/opengisch/qgis-plugin-ci/issues/357#issuecomment-3753400395_