LibJS: Early return from Date string parsing on empty string

Loading Ladybird on Github results in 37 debug logs about being unable
to parse an empty Date string. This log is intended to catch Date
formats we do not support to detect web compatability problems, which
makes this case not particuarly useful to log.

Instead of trying to parse all of the different date formats and
logging that the string is not valid, let's just return NAN immediately.

Userland/Libraries/LibJS/Runtime/DateConstructor.cpp

@@ -152,6 +152,9 @@ static double parse_simplified_iso8601(ByteString const& iso_8601)
 
 static double parse_date_string(VM& vm, ByteString const& date_string)
 {
+    if (date_string.is_empty())
+        return NAN;
+
     auto value = parse_simplified_iso8601(date_string);
     if (isfinite(value))
         return value;