PHP 8.5 compatibility

[emil-totev]

After upgrading to php 8.5 I started getting warnings:
Deprecated: Using null as an array offset is deprecated, use an empty string instead in ... \goat1000\svggraph\Javascript.php on line 186

[goat1000]

Commit 8c5e750 should fix this warning and a few others I found.

There might be more warnings like this, as it will happen whenever $some_array[$key] is used with a null key value.

If I don't find any more warnings to fix I'll tag a release next week.

[goat1000]

I've tagged this as 3.20.3 as I couldn't find any more warnings.