tiny extra optimization for SkColor4fXformer

When dst is null we can't hit the Equals() fast path,
as Equals() says only null == null.  Instead treat
null dst as src, i.e. skip the transform.

This should be an observable no-op... SkConvertPixels
will recognize null dst == src and do no color transform.
This CL just avoids the extra copy.

Change-Id: I7c1983d5db733a2dd036d84c2ff91add136e1c68
Reviewed-on: https://skia-review.googlesource.com/c/161587
Commit-Queue: Mike Klein <mtklein@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
Auto-Submit: Mike Klein <mtklein@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>

src/shaders/gradients/SkGradientShader.cpp

@@ -462,7 +462,7 @@ SkColor4fXformer::SkColor4fXformer(const SkColor4f* colors, int colorCount,
                                    SkColorSpace* src, SkColorSpace* dst) {
     fColors = colors;
 
-    if (!SkColorSpace::Equals(src, dst)) {
+    if (dst && !SkColorSpace::Equals(src, dst)) {
         fStorage.reset(colorCount);
 
         auto info = SkImageInfo::Make(colorCount,1, kRGBA_F32_SkColorType, kUnpremul_SkAlphaType);