bug(swift): empty version of package

[DmitriyLewen]

Description

There are cases when Version field of Pin is empty.
In this case Branch field is used.
e.g.:

    {
          "identity" : "swift-ogg",
          "kind" : "remoteSourceControl",
          "location" : "https://github.com/element-hq/swift-ogg",
          "state" : {
            "branch" : "0.0.1",
            "revision" : "e9a9e7601da662fd8b97d93781ff5c60b4becf88"
          }
    }

We need to check Branch field or skip this package (with log message) if Version and Branch are empty.

Discussed in #6165