fix: radialGradient r={0} (#2271)

# Summary

* Fixed crash on Android when `<RadialGradient r={0}>`.
* Fixed render issue on Android and iOS when radius is zero. According
to [MDN
Docs](https://developer.mozilla.org/en-US/docs/Web/SVG/Attribute/r#radialgradient)
> A value of lower or equal to zero will cause the area to be painted as
a single color using the color and opacity of the last gradient
`<stop>`.

## Test Plan

Test is available in `TestsExample` as `Test2170`

## Compatibility

| OS      | Implemented |
| ------- | :---------: |
| iOS     |    ✅    |
| Android |    ✅    |


Fixes #2170

apple/Brushes/RNSVGPainter.mm

@@ -232,6 +232,15 @@ void PatternFunction(void *info, CGContextRef context)
   CGFloat rx = [self getVal:[_points objectAtIndex:2] relative:width];
   CGFloat ry = [self getVal:[_points objectAtIndex:3] relative:height];
 
+  if (rx <= 0 || ry <= 0) {
+    // Gradient with radius = 0 should be rendered as solid color of the last stop
+    rx = width;
+    ry = height;
+    CGGradientRelease(gradient);
+    NSArray<NSNumber *> *gradientArray = @[_colors.firstObject, _colors.lastObject, _colors[_colors.count-2], _colors.lastObject];
+    gradient = CGGradientRetain([RCTConvert RNSVGCGGradient:gradientArray]);
+  }
+    
   double ratio = ry / rx;
 
   CGFloat fx = [self getVal:[_points objectAtIndex:0] relative:width] + offsetX;