Fix nil type checks

Author: antonmedv

Diff for: checker/checker_test.go

@@ -733,3 +733,15 @@ func TestCheck_CallFastTyped_Method(t *testing.T) {
 	require.False(t, tree.Node.(*ast.CallNode).Fast)
 	require.Equal(t, 42, tree.Node.(*ast.CallNode).Typed)
 }
+
+func TestCheck_works_with_nil_types(t *testing.T) {
+	env := map[string]interface{}{
+		"null": nil,
+	}
+
+	tree, err := parser.Parse("null")
+	require.NoError(t, err)
+
+	_, err = checker.Check(tree, conf.New(env))
+	require.NoError(t, err)
+}

Diff for: checker/types.go

@@ -204,6 +204,9 @@ func fetchField(t reflect.Type, name string) (reflect.StructField, bool) {
 }
 
 func deref(t reflect.Type) (reflect.Type, bool) {
+	if t == nil {
+		return nil, false
+	}
 	if t.Kind() == reflect.Interface {
 		return t, true
 	}