Fix lastIndexOf behavior against an empty string in strings extension (#1173)

* Fix lastIndexOf behavior against an empty string in strings extension

Author: l46kok

ext/strings.go

@@ -607,6 +607,10 @@ func lastIndexOf(str, substr string) (int64, error) {
 	if substr == "" {
 		return int64(len(runes)), nil
 	}
+
+	if len(str) < len(substr) {
+		return -1, nil
+	}
 	return lastIndexOfOffset(str, substr, int64(len(runes)-1))
 }
 

ext/strings_test.go

@@ -53,6 +53,7 @@ var stringTests = []struct {
 	{expr: `'hello wello'.indexOf('ello', 6) == 7`},
 	{expr: `'hello wello'.indexOf('elbo room!!') == -1`},
 	{expr: `'hello wello'.indexOf('elbo room!!!') == -1`},
+	{expr: `''.lastIndexOf('@@') == -1`},
 	{expr: `'tacocat'.lastIndexOf('') == 7`},
 	{expr: `'tacocat'.lastIndexOf('at') == 5`},
 	{expr: `'tacocat'.lastIndexOf('none') == -1`},