Simplify optional input handling in SkMixerColorFilter

When the first input is null, swap with the second - this allows the
implementation to assume CF0 != null.

TBR=
Change-Id: I9b3d0aeecaffed3b56f8956138305b17d33eb1de
Reviewed-on: https://skia-review.googlesource.com/c/193164
Reviewed-by: Florin Malita <fmalita@chromium.org>
Commit-Queue: Florin Malita <fmalita@chromium.org>