All options optional in one of sets

**Describe the bug**
I have two parameter sets. One has only one option, bool list, works fine:

`mytool new --list # works fine`

The other set works fine, while it has at least one required option

`mytool new --template "default"`

But I want all the options in my second set to be optional. I'd like to run

`mytool new`

and expect the second set is picked up, with "template" option set to "default". I cannot achieve it.

**To Reproduce**
https://dotnetfiddle.net/VE1xFs

**Expected behavior**
When one of the sets has all options optional, with defaults, it is automatically picked when no command-line arguments are provided.

Start with the linked .NET Fiddle reproducer and compare parameter-set selection when no arguments are supplied with selection when a required option is present. Trace how the parser handles an all-optional set and its defaults. Done means `mytool new` selects that set with `template` set to `default`, while the existing required-option and `--list` cases continue to work.

Written by the indexing model from the issue text.