[CP-beta][Impeller] handle fill polylines with zero area. (#51948)

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### Issue Link:
What is the link to the issue this cherry-pick is addressing?

flutter/flutter#146362

### Changelog Description:
Explain this cherry pick in one line that is accessible to most Flutter developers. See [best practices](https://github.com/flutter/flutter/wiki/Hotfix-Documentation-Best-Practices) for examples

Fixes Impeller crash when rendering zero area filled shapes.

### Impact Description:
What is the impact (ex. visual jank on Samsung phones, app crash, cannot ship an iOS app)? Does it impact development (ex. flutter doctor crashes when Android Studio is installed), or the shipping production app (the app crashes on launch)

Native crash in Impeller renderer.

### Workaround:
Is there a workaround for this issue?

No

### Risk:
What is the risk level of this cherry-pick?

### Test Coverage:
Are you confident that your fix is well-tested by automated tests?

### Validation Steps:
What are the steps to validate that this fix works?

Run the included test

Author: flutteractionsbot

impeller/tessellator/tessellator.cc

@@ -187,6 +187,9 @@ std::vector<Point> Tessellator::TessellateConvex(const Path& path,
                           [this](Path::Polyline::PointBufferPtr point_buffer) {
                             point_buffer_ = std::move(point_buffer);
                           });
+  if (polyline.points->size() == 0) {
+    return output;
+  }
 
   output.reserve(polyline.points->size() +
                  (4 * (polyline.contours.size() - 1)));

impeller/tessellator/tessellator_unittests.cc

@@ -480,6 +480,19 @@ TEST(TessellatorTest, FilledRoundRectTessellationVertices) {
        Rect::MakeXYWH(5000, 10000, 2000, 3000), {50, 70});
 }
 
+TEST(TessellatorTest, EarlyReturnEmptyConvexShape) {
+  // This path is not technically empty (it has a size in one dimension),
+  // but is otherwise completely flat.
+  auto tessellator = std::make_shared<Tessellator>();
+  PathBuilder builder;
+  builder.MoveTo({0, 0});
+  builder.MoveTo({10, 10}, /*relative=*/true);
+
+  auto points = tessellator->TessellateConvex(builder.TakePath(), 3.0);
+
+  EXPECT_TRUE(points.empty());
+}
+
 #if !NDEBUG
 TEST(TessellatorTest, ChecksConcurrentPolylineUsage) {
   auto tessellator = std::make_shared<Tessellator>();