Do not run TryParseExact on null dates

Author: pakrym

src/Microsoft.Net.Http.Headers/HttpRuleParser.cs

@@ -241,6 +241,11 @@ internal static string DateToString(DateTimeOffset dateTime)
 
         internal static bool TryStringToDate(string input, out DateTimeOffset result)
         {
+            if (input == null)
+            {
+                return false;
+            }
+
             // Try the various date formats in the order listed above.
             // We should accept a wide verity of common formats, but only output RFC 1123 style dates.
             if (DateTimeOffset.TryParseExact(input, DateFormats, DateTimeFormatInfo.InvariantInfo,