Fixed: U4-4119 - Export ContentType fails for Generic Properties

Adds Null checking for PropertyGroupID so that an exception is avoided
when the propertyType is in the Generic Properties group (and has no ID
assigned)
Export Template fails when template has no master value
Revert "Export Template fails when template has no master value"

This reverts commit a2905c13f6f42095e07fa20ae402e514400cecd0.
Single DataType import Fix

Importing a file with a single data type failed, because the DataType
element is at the root when exported.
Fix Importing Macro when macro already exists

ParseMacroElement called by ImportMacro was no checking for to see if
the macro or properties already exists - caused SQL errors on import of
macro. Added check to look for existing macro, and only add new
properties on import (in line with other import methods)
Fix Template Import when template has no master

Importing a template with no master template (i.e when it's at the root)
caused an error. Added check for MasterTemplateId != null - (tried
.IsValueCreated but that generated a object not set exception)
Merge branch 'KevinJump-7.0.2' into 7.1.0