core: Add test and fix for element with empty list

The incldued test previously caused a panic, it now returns an error
message explaining the issue.

config/interpolate_funcs.go

@@ -667,6 +667,9 @@ func interpolationFuncElement() ast.Function {
 		ReturnType: ast.TypeString,
 		Callback: func(args []interface{}) (interface{}, error) {
 			list := args[0].([]ast.Variable)
+			if len(list) == 0 {
+				return nil, fmt.Errorf("element() may not be used with an empty list")
+			}
 
 			index, err := strconv.Atoi(args[1].(string))
 			if err != nil || index < 0 {

config/interpolate_funcs_test.go

@@ -952,6 +952,7 @@ func TestInterpolateFuncElement(t *testing.T) {
 		Vars: map[string]ast.Variable{
 			"var.a_list":       interfaceToVariableSwallowError([]string{"foo", "baz"}),
 			"var.a_short_list": interfaceToVariableSwallowError([]string{"foo"}),
+			"var.empty_list":   interfaceToVariableSwallowError([]interface{}{}),
 		},
 		Cases: []testFunctionCase{
 			{
@@ -980,6 +981,13 @@ func TestInterpolateFuncElement(t *testing.T) {
 				true,
 			},
 
+			// Empty list should fail
+			{
+				`${element(var.empty_list, 0)}`,
+				nil,
+				true,
+			},
+
 			// Too many args
 			{
 				`${element(var.a_list, "0", "2")}`,