The current implementation of HierarchicalStringList allows the following:

   FOO.bar = [
       'foo',
       'bar',
   ]

while

   FOO.bar += [
       'foo',
       'bar',
   ]

would be invalid because of the StrictOrderingOnAppendList enforcement.

It also allows to overwrite the entire list with a subsequent

   FOO.bar = [
	'baz',
   ]

while we've explicitly forbidden such things for every other list.

While in the vicinity, fix HierarchicalStringList._get_export_variable to not
call the HierarchicalStringList constructor uselessly.