Do not send None arguments to the front-end

Author: martinRenou

examples/introduction.ipynb

@@ -113,7 +113,7 @@
    "metadata": {},
    "outputs": [],
    "source": [
-    "c.fill_text('Hello World!!', 50, 50, 50)"
+    "c.fill_text('Hello World!!', 50, 50)"
    ]
   },
   {

ipycanvas/canvas.py

@@ -126,7 +126,7 @@ def _on_set_attr(self, change):
         self._send_command(command)
 
     def _send_canvas_command(self, name, *args):
-        self._send_command({'name': name, 'args': args})
+        self._send_command({'name': name, 'args': [arg for arg in args if arg is not None]})
 
     def _send_command(self, command):
         if self.caching: