Further Enhancements to text-shadow parsing

[Sub6Resources]

text-shadow: rgb(2, 0, 171) 1px 0px, rgb(2, 0, 171) -1px 0px, rgb(2, 0, 171) 0px 1px, rgb(2, 0, 171) 0px -1px;

Not supported this style?

Null check operator used on a null value

The relevant error-causing widget was: 
  Html Html:file:///Users/shingle/ws/android/foresightnews/lib/pages/detail/articleDetail.dart:207:30
When the exception was thrown, this was the stack: 
#0      ExpressionMapping.expressionToTextShadow (package:flutter_html/src/css_parser.dart:901:100)
#1      declarationsToStyle.<anonymous closure> (package:flutter_html/src/css_parser.dart:339:48)

_Originally posted by @abcdsxg in https://github.com/Sub6Resources/flutter_html/issues/834#issuecomment-1223410739_