When importing antora.yml, prerelease should only be alternated when version is specified

If the identify of the component version is being updated, the prerelease key should only be removed if the value of the version key is specified and differs from the current version. In other words, version and prerelease form a pair. If the version is not specified, or is unchanged, both should be left as is. If the version is specified, then the prerelease should be cleared unless specified.

It should be possible to clear the prerelease explicitly by setting the value to null.