FIX: Don't set group for pasted composite binding

[jamre]

When pasting a copied composite binding in the asset editor, if the editor is filtering to show only one scheme, the group will get set to that scheme on the composite binding when it should be left empty.

This PR skips setting the group if the binding is a composite.

[Rene-Damm]

PSA: We're super slow ATM in getting external PRs in. Reason is we have no good process in place yet and we're figuring out some way to handle external PRs in a reasonable fashion. One problem, for example, is that we can't run our CI directly on these PRs so we have to manually pull in and run the changes. Please stay tuned. The PRs are definitely appreciated.

[unity-cla-assistant]

All committers have signed the CLA.

[jimon]

Hi thanks for the PR, we will merge it as part of #1604