Fix U4-4119 - Export Fails for Generic Properties

Adds null checking for PropertyGroupID on export so that properties with
no group (tab) don't throw an exception.
PackageService: Fixes Single import for DataTypes

Importing a file with a single data type failed, because the DataType
element is at the root when exported.
Merge pull request #298 from KevinJump/6.2.0--PackageServiceFixes

6.2.0  package service fixes