let data-attribute be optional for attr_value

[thet]

attr_value must be called with the attribute-key, the widget instance and a data-dict. this data-dict is not always available, like in cssid: db9619f#L1R152

the 'structural' attribute would be better queried by attr_value('structural', widget, data) to cover callables too, but the data-dict is not available.

attr_value('structural', widget, {}) might be possible, but that seems like a misuse of the API. passing the data-dict to all cssid instances is another option. but maybe the data-dict is not always needed, like in the cssid case. so the call to attr(widget, data) in attr_value would be ok without data. in this case, the attr_value function signature should look like:
attr_value(key, widget, data={}, default=None)

[rnixx]

There is a similar issue with property callables in preprocessors where data is not available yet.
+1 for the signature change