Improve oauth2 scope token handling

[wxiaoguang]

No description provided.

[marcellmars]

i'm sure @wxiaoguang aligned better with the gitea's style. this is the minimal diff doing the same:

modified   services/oauth2_provider/access_token.go
@@ -83,7 +83,7 @@ func GrantAdditionalScopes(grantScopes string) auth.AccessTokenScope {
 
 	var tokenScopes []string
 	for _, tokenScope := range strings.Split(grantScopes, " ") {
-		if slices.Index(scopesSupported, tokenScope) == -1 {
+		if slices.Index(scopesSupported, tokenScope) == -1 && tokenScope != "" {
 			tokenScopes = append(tokenScopes, tokenScope)
 		}
 	}

[wxiaoguang]

Applied your suggestion in a9415a6, major changes:

1. add the empty scope check
2. add if len(accessScopes) check before normalizing , now we have a clear code path to distinguish "empty token" and "invalid token" case (err != nil)
3. fine tune the comments, leave the potential breaking change to the future.