macOS: Prevent an empty RectangleList from incorrectly filling a graphics context

modules/juce_graphics/native/juce_CoreGraphicsContext_mac.mm

@@ -593,6 +593,9 @@ void CoreGraphicsContext::drawLine (const Line<float>& line)
 
 void CoreGraphicsContext::fillRectList (const RectangleList<float>& list)
 {
+    if (list.isEmpty())
+        return;
+
     std::vector<CGRect> rects;
     rects.reserve ((size_t) list.getNumRectangles());