Return singleton enumerators from IEnumerable.GetEnumerator for empty collections...

Return singleton enumerators from IEnumerable.GetEnumerator for empty collections (#82499)

* Return singleton enumerators from IEnumerable.GetEnumerator for empty collections

Change the `IEnumerable<T>.GetEnumerator()` implementations on our core collection types to special-case Count==0 in order to return a single enumerator instead of allocating one a new each time.  This saves an allocation when enumerating these collections via the interface in exchange for an extra length check as part of GetEnumerator.

* Address PR feedback

- Create helper function for empty enumerator
- Add tests for singletons

* Fix a few tests