Empty array of vars with empty domain makes model unsat

The following illustrates it. It happened in an automatically generated model. IMHO, this should not make the model unsat, because there are no variables in the domain.
```
$ cat > foo.mzn
array[1..0] of var 1..0: x;
$ minizinc foo.mzn

  WARNING: model inconsistency detected: domain is empty
  /Users/matsc/c2radio/foo.mzn:1:
  in variable declaration for 'x'
=====UNSATISFIABLE=====
```
