LibWeb: Fix null dereference when assigning an ImageStyleValue via JS

When parsing a CSS value in the context of a CSSStyleDeclaration
camelCase property setter, we don't necessarily have a Document to
provide the CSS parser for context.

So the parser can't go assuming that there's always a Document in the
ParsingContext. And ImageStyleValue can't go assuming that there's
always a Document either. This will require some more work to get things
right, I'm just patching up the null dereference for now.

Userland/Libraries/LibWeb/CSS/Parser/Parser.cpp

@@ -1727,7 +1727,7 @@ RefPtr<StyleValue> Parser::parse_image_value(ParsingContext const& context, Styl
 {
     auto url = parse_url_function(context, component_value);
     if (url.has_value())
-        return ImageStyleValue::create(url.value(), *context.document());
+        return ImageStyleValue::create(url.value(), context.document());
     // FIXME: Handle gradients.
 
     return {};