LibTextCodec+Everywhere: Return Optional<Decoder&> from decoder_for()

Author: https://github.com/AtkinsSJ
Commit: f2a9426885
Pull-request: https://github.com/SerenityOS/serenity/pull/17511
Reviewed-by: https://github.com/ADKaster
Reviewed-by: https://github.com/nico

Userland/Libraries/LibWeb/Encoding/TextDecoder.cpp

@@ -15,7 +15,7 @@ namespace Web::Encoding {
 WebIDL::ExceptionOr<JS::NonnullGCPtr<TextDecoder>> TextDecoder::construct_impl(JS::Realm& realm, DeprecatedFlyString encoding)
 {
     auto decoder = TextCodec::decoder_for(encoding);
-    if (!decoder)
+    if (!decoder.has_value())
         return WebIDL::SimpleException { WebIDL::SimpleExceptionType::TypeError, DeprecatedString::formatted("Invalid encoding {}", encoding) };
 
     return MUST_OR_THROW_OOM(realm.heap().allocate<TextDecoder>(realm, realm, *decoder, move(encoding), false, false));