Handle a common case in Foo.lerp(a, b, t): if a and b are identical, then just return a. This avoids needlessly constructing a new Foo and recursively lerping all of its properties. If a and b refer to the same const object, then we also preserve their singleton identity.

This PR is similar to #120829

All of the lerp methods in the began library began with:

if (a == null && b == null) {
  return null;
}

This PR safely generalizes the short-circuit to:

if (identical(a, b)) {
  return a;
}