ValueError: could not convert string to float: 'null'

Hello,

I get the below error when I try to convert my svg data to png
```
File [~\pythonEnv\lib\site-packages\cairosvg\surface.py:410](http://localhost:8888/lab/tree/GraphDL/~/pythonEnv/lib/site-packages/cairosvg/surface.py#line=409), in Surface.draw(self, node)
407 pass
409 # Get stroke and fill opacity
--> 410 stroke_opacity = float(node.get('stroke-opacity', 1))
411 fill_opacity = float(node.get('fill-opacity', 1))
412 if opacity < 1 and not node.children:

ValueError: could not convert string to float: 'null'
```

I was wondering how I can fix it?

best

Start at cairosvg/surface.py around line 410, where the traceback shows stroke opacity is converted during SVG-to-PNG rendering. Obtain or recreate the SVG that produces the 'null' value, then trace how that attribute reaches this code. Done means the reported SVG converts successfully and the behavior is covered by a regression test.

Written by the indexing model from the issue text.