[sksg] Fix uninitialized DashEffect attribute

All SkSG attributes require a default initializer.

TBR=
Change-Id: I15ee6ea56f1d07375af5a623e95321bd80da2e68
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/321470
Reviewed-by: Florin Malita <fmalita@chromium.org>
Commit-Queue: Florin Malita <fmalita@google.com>
Commit-Queue: Florin Malita <fmalita@chromium.org>

modules/sksg/include/SkSGGeometryEffect.h

@@ -114,7 +114,7 @@ private:
     SkPath onRevalidateEffect(const sk_sp<GeometryNode>&) override;
 
     std::vector<float> fIntervals;
-    float              fPhase;
+    float              fPhase = 0;
 
     using INHERITED = GeometryEffect;
 };