Ensure that the stroke_width property of SVGMobject is not set to None. (#4319)

This change fixes issue 4311.

Author: henrikmidtiby

manim/mobject/svg/svg_mobject.py

@@ -125,6 +125,8 @@ def __init__(
         self.stroke_color = stroke_color
         self.stroke_opacity = stroke_opacity
         self.stroke_width = stroke_width
+        if self.stroke_width is None:
+            self.stroke_width = 0
 
         if svg_default is None:
             svg_default = {