[BUG] Inline style with "text-shadow" in some case will parse failed.

[windows7lake]

Describe the bug:
Using the code as follow will parse failed, and the code was generated by some kind of editor.

text-shadow: #FC0 1px 0 10px;
text-shadow: red 2px 5px;
text-shadow: none;

report that

======== Exception caught by widgets library =======================================================
The following _CastError was thrown building HtmlParser-[GlobalKey#fb3cb](dirty, dependencies: [_LocalizationsScope-[GlobalKey#d5a1c], _InheritedTheme]):
Null check operator used on a null value

The relevant error-causing widget was: 
  HtmlParser-[GlobalKey#fb3cb] file:///Users/xxxx/Workspace/FlutterWorker/flutter_html/lib/flutter_html.dart:159:14
When the exception was thrown, this was the stack: 
#0      ExpressionMapping.expressionToTextShadow (package:flutter_html/src/css_parser.dart:832:88)
#1      declarationsToStyle.<anonymous closure> (package:flutter_html/src/css_parser.dart:299:48)
#2      _LinkedHashMapMixin.forEach (dart:collection-patch/compact_hash.dart:397:8)
#3      declarationsToStyle (package:flutter_html/src/css_parser.dart:14:16)
#4      inlineCssToStyle (package:flutter_html/src/css_parser.dart:312:12)
...
====================================================================================================

HTML to reproduce the issue:

<p style="text-shadow: red 2px 5px;">Test</p>

Just add this code to example.

Html widget configuration:
the same as example.

Expected behavior:
Parse correct. With some code like "text-shadow: red 2px 5px;", it can be parsed as follow:

[erickok]

Should be fixed in 2.2.0.

[abcdsxg]

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)

[Sub6Resources]

@abcdsxg It's possible the rgb keyword still might have some issues, as well as the comma separated list of text shadows. I've opened a new issue for this in #1130.