LibWeb: Omit initial values from background CSS property serialization

Author: https://github.com/Calme1709
Commit: 0a53aaa3b6
Pull-request: https://github.com/LadybirdBrowser/ladybird/pull/4990
Reviewed-by: https://github.com/AtkinsSJ ✅
Reviewed-by: https://github.com/konradekk

Tests/LibWeb/Text/expected/background-style-declaration-cssText-crash.txt

@@ -1,2 +1,2 @@
-:root { background: transparent none 1px 1px auto auto repeat scroll padding-box border-box; }
+:root { background: 1px 1px; }
 PASS (didn't crash)

Tests/LibWeb/Text/expected/css/CSSStyleProperties-all-supported-properties-and-default-values.txt

@@ -174,7 +174,7 @@ All supported properties and their default values exposed from CSSStylePropertie
 'aspect-ratio': 'auto'
 'backdropFilter': 'none'
 'backdrop-filter': 'none'
-'background': 'rgba(0, 0, 0, 0) none 0% 0% auto auto repeat scroll padding-box border-box'
+'background': 'none'
 'backgroundAttachment': 'scroll'
 'background-attachment': 'scroll'
 'backgroundBlendMode': 'normal'