[dev] DataTree::getAttributes() empty check

[Chuck Hagenbuch]

Quoting Duck <duck at obala.net>:

> Sometimes happens that we pass empty cid value to DataTree::getAttributes()
> method. For example while listing Agora Forms where there are no messages
> posted in them. So is a waste of resources to go in SQL to try to get
> unexisting messages data. Patch check this.

Committed, thanks.

-chuck

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