LibWeb: Serialize grid-area as auto if all sub-properties are auto

Author: https://github.com/tcl3
Commit: c029a9c98c
Pull-request: https://github.com/LadybirdBrowser/ladybird/pull/4004
Reviewed-by: https://github.com/AtkinsSJ ✅

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

@@ -366,8 +366,8 @@ All supported properties and their default values exposed from CSSStylePropertie
 'font-width': 'normal'
 'gap': 'normal'
 'grid': ''
-'gridArea': ''
-'grid-area': ''
+'gridArea': 'auto'
+'grid-area': 'auto'
 'gridAutoColumns': 'auto'
 'grid-auto-columns': 'auto'
 'gridAutoFlow': 'row'