Fixed NullPointerException Bug

`RequestTemplate.header(name, values)` method has two overloaded implementations.
In the case where `values` has type `Iterable<String>`, `values` is guarded against `null`.
This does not happen when `values has type `String...`, which is fixed by this commit

core/src/main/java/feign/RequestTemplate.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2012-2023 The Feign Authors
+ * Copyright 2012-2024 The Feign Authors
  *
  * Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except
  * in compliance with the License. You may obtain a copy of the License at
@@ -703,6 +703,10 @@ public Map<String, Collection<String>> queries() {
    * @see RequestTemplate#header(String, Iterable)
    */
   public RequestTemplate header(String name, String... values) {
+    if (values == null) {
+      return appendHeader(name, new Collections.emptyList());
+    }
+
     return header(name, Arrays.asList(values));
   }