Make package migration resiliant to unversioned packages.

Sometimes UPM returns package info without version information
which breaks version parsing logic. This commit gracefully handles
missing version information by setting the package version to 0.

Change-Id: I6014dd1dac105c79268f5b646e5fd2723992de3d

Author: Stewart Miles

source/UnityPackageManagerResolver/src/PackageMigrator.cs

@@ -1052,7 +1052,9 @@ public static HashSet<string> GetVersionHandlerPackageNames(
     /// <returns>Numeric version of the package that can be compared with Version Handler package
     /// versions.</returns>
     public static long CalculateVersion(this UnityPackageManagerClient.PackageInfo packageInfo) {
-        return VersionHandlerImpl.FileMetadata.CalculateVersion(packageInfo.Version);
+        var version = packageInfo.Version;
+        return version != null ?
+            VersionHandlerImpl.FileMetadata.CalculateVersion(version) : 0;
     }
 }