Do not run custom header parsers on null header values (#678)

Author: pakrym

src/Microsoft.AspNetCore.Http.Extensions/HeaderDictionaryTypeExtensions.cs

@@ -176,18 +176,19 @@ internal static T Get<T>(this IHeaderDictionary headers, string name)
             }
 
             object temp;
-            if (KnownParsers.TryGetValue(typeof(T), out temp))
-            {
-                var func = (Func<string, T>)temp;
-                return func(headers[name]);
-            }
-
             var value = headers[name];
+
             if (StringValues.IsNullOrEmpty(value))
             {
                 return default(T);
             }
 
+            if (KnownParsers.TryGetValue(typeof(T), out temp))
+            {
+                var func = (Func<string, T>)temp;
+                return func(value);
+            }
+
             return GetViaReflection<T>(value.ToString());
         }
 
@@ -199,18 +200,19 @@ internal static IList<T> GetList<T>(this IHeaderDictionary headers, string name)
             }
 
             object temp;
-            if (KnownListParsers.TryGetValue(typeof(T), out temp))
-            {
-                var func = (Func<IList<string>, IList<T>>)temp;
-                return func(headers[name]);
-            }
-
             var values = headers[name];
+
             if (StringValues.IsNullOrEmpty(values))
             {
                 return null;
             }
 
+            if (KnownListParsers.TryGetValue(typeof(T), out temp))
+            {
+                var func = (Func<IList<string>, IList<T>>)temp;
+                return func(values);
+            }
+
             return GetListViaReflection<T>(values);
         }