Avoid panic when used with the recover middleware (#14)

* Avoid panic when used with the recover middleware

When using a custom log handler and the recover middleware,
err is (sometimes?) nil.

---------

Co-authored-by: Samuel Berthe <dev@samuel-berthe.fr>

middleware.go

@@ -215,10 +215,18 @@ func NewWithConfig(logger *slog.Logger, config Config) echo.MiddlewareFunc {
 			msg := "Incoming request"
 			if status >= http.StatusInternalServerError {
 				level = config.ServerErrorLevel
-				msg = err.Error()
+				if err != nil {
+					msg = err.Error()
+				} else {
+					msg = http.StatusText(status)
+				}
 			} else if status >= http.StatusBadRequest && status < http.StatusInternalServerError {
 				level = config.ClientErrorLevel
-				msg = err.Error()
+				if err != nil {
+					msg = err.Error()
+				} else {
+					msg = http.StatusText(status)
+				}
 			}
 
 			logger.LogAttrs(c.Request().Context(), level, msg, attributes...)