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

Nearly all of the lerp methods in the painting library began with:

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

This PR safely generalizes the short-circuit to:

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

There were a few lerp methods that did not follow this pattern.

• Gradient.lerp included this short-circuit after handling special cases where either a or b where non-null. AFAICT, moving the short-circuit to the beginning of the method does not change its semantics.
• The ShapeBorder, OutlinedBorder, and BoxBorder lerp methods did not include a short-circuit however they did return null for lerp(null, null, t) and so the identical seems to only be an improvement.
• BorderSide.lerp's a and b parameters are non-nullable.