New GN: Is Connected

A new Geometry Node to detect whether optional complex-type Group Input sockets have been connected/supplied a suitable source of data.

For example, if my geo node network can handle either a Curve or a Mesh but requires different processing, I want to be able to process them differently and not combine the results. If I just use Join Geometry, I might be left with vestigial results from the undesired empty source. (For example, Curve to Mesh with an empty Profile gives points and edges geometry but I would want it to produce no geometry in some cases.)

Or if the user leaves a Material empty, I would like to use an internally built-in Material as a default which cannot be supplied by the Group Inputs interface.

The drop-down should select the input type, just like the Switch node does. Since "simple" types can have a default value supplied by the Group Input, these input types do not need to be supported by this Is Connected node. I think the value is for the more complicated values like Object, Geometry, Material, Texture, etc., which cannot supply a default value.

An alternative UI would not include an argument socket, but the drop-down simply select from all suitable Group Input socket names. This would have trouble distinguishing between two identically-named Group Input sockets, which is currently allowed.