Previously, interpolation of multi-variables was returning an empty
variable if the resource count was 0. The empty variable was defined as
TypeString, Value "". This means that empty resource counts fail type
checking for interpolation functions which operate on lists.
Instead, return an empty list if the count is 0. A context test tests
this against further regression. Also add a regression test covering the
case of a single count multi-variable.
In order to make the context testing framework deal with this change it
was necessary to special case empty lists in the test diff function.
Fixes#7002
