Enable govet check for empty strings (#16785)

Adds the check for https://github.com/mattermost/mattermost-server/issues/16674
in the Makefile

```release-note
NONE
```

web/saml.go

@@ -107,7 +107,7 @@ func completeSaml(c *Context, w http.ResponseWriter, r *http.Request) {
 	hasRedirectURL := false
 	if val, ok := relayProps["redirect_to"]; ok {
 		redirectURL = val
-		hasRedirectURL = len(val) > 0
+		hasRedirectURL = val != ""
 	}
 
 	handleError := func(err *model.AppError) {