LibWeb: Fix issues with CSS attribute selector handling

This is three small, related changes:

1. Element::has_attribute() now returns true if the attribute exists but
has no value. (eg, `<div foo />` -> `has_attribute("foo")`)

2. SelectorEngine::matches_attribute() now makes sure there is a first
segment before comparing it, fixing a crash.

3. CSS::Parser now converts attribute names in attribute selectors to
lowercase, to match the expectations of the rest of the system.
Converting to lowercase is not always correct, depending on language,
but since we only currently support HTML, and that expects them to be
case-insensitive, it is fine for now.

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

@@ -416,7 +416,12 @@ Result<Selector::SimpleSelector, Parser::SelectorParsingResult> Parser::parse_si
             .type = Selector::SimpleSelector::Type::Attribute,
             .attribute = {
                 .match_type = Selector::SimpleSelector::Attribute::MatchType::HasAttribute,
-                .name = attribute_part.token().ident(),
+                // FIXME: Case-sensitivity is defined by the document language.
+                // HTML is insensitive with attribute names, and our code generally assumes
+                // they are converted to lowercase, so we do that here too. If we want to be
+                // correct with XML later, we'll need to keep the original case and then do
+                // a case-insensitive compare later.
+                .name = attribute_part.token().ident().to_lowercase_string(),
             }
         };