solve potential nil error for str.When

support/str/str.go

@@ -804,12 +804,14 @@ func (s *String) Upper() *String {
 }
 
 // When returns the String instance with the given callback applied if the given condition is true.
-// If the condition is false, the fallback callback is applied.(if provided)
+// If the condition is false, the fallback callback is applied (if provided).
 func (s *String) When(condition bool, callback ...func(*String) *String) *String {
 	if condition {
-		return callback[0](s)
+		if len(callback) > 0 && callback[0] != nil {
+			return callback[0](s)
+		}
 	} else {
-		if len(callback) > 1 {
+		if len(callback) > 1 && callback[1] != nil {
 			return callback[1](s)
 		}
 	}