create empty no selection syntax provided to group command

[dave-connors-3]

closes #77

if no argument provided for selecting nodes, the group command will create an empty group in the project

[graciegoheen]

Why have it fail vs. create an empty group?

[dave-connors-3]

open to it! what's the benefit of an empty group?

[graciegoheen]

Maybe I want to start by creating an empty group and manually add models to it

[dave-connors-3]

@nicholasyager thought on this?

[nicholasyager]

🤔 Putting on my user shoes, I would expect that not supplying a selector would result in no selected models in the group. I do not think that people would intuitively expect all models to be added to a single group if no selector is provided.

[dave-connors-3]

@nicholasyager @graciegoheen updated the logic to simply create an empty group without selection syntax!

[nicholasyager]

@dave-connors-3 This change looks both necessary and sufficient. Kudos for catching the test updates, too!