core: Fix interpolation of unknown multi-variables

This commit test "TestContext2Input_moduleComputedOutputElement"
by ensuring that we treat a count of zero and non-reified resources
independently rather than returning an empty list for both, which
results in an interpolation failure when using the element function or
indexing.

terraform/interpolate.go

@@ -162,7 +162,6 @@ func (i *Interpolater) valueModuleVar(
 		} else {
 			// Same reasons as the comment above.
 			result[n] = unknownVariable()
-
 		}
 	}
 
@@ -485,11 +484,15 @@ func (i *Interpolater) computeResourceMultiVariable(
 			err)
 	}
 
-	// If we have no module in the state yet or count, return empty
+	// If count is zero, we return an empty list
-	if module == nil || len(module.Resources) == 0 || count == 0 {
+	if count == 0 {
 		return &ast.Variable{Type: ast.TypeList, Value: []ast.Variable{}}, nil
 	}
 
+	// If we have no module in the state yet or count, return unknown
+	if module == nil || len(module.Resources) == 0 {
+		return &unknownVariable, nil
+	}
 	var values []string
 	for j := 0; j < count; j++ {
 		id := fmt.Sprintf("%s.%d", v.ResourceId(), j)