Every ::set_atts() implementation used to check on its own whether the
property bag is empty (common for "plain" SVG elements with no defined
attributes?) before actually querying the property bag.

Now we do that check in a single place, in the toplevel node_set_atts()
helper, instead of having each node type do it on its own.

This means that ::set_atts() implementation which do "validation" work
or any other post-initialization after the attributes have been set,
will have to support their ::set_atts() not being called at all if there
are no attributes in the first place.