Fix issue with null headers

Author: slinkydeveloper

common/src/main/java/dev/restate/common/RequestBuilder.java

@@ -18,7 +18,7 @@ public interface RequestBuilder<Req, Res> extends Request<Req, Res> {
    * @param idempotencyKey Idempotency key to attach in the request.
    * @return this instance, so the builder can be used fluently.
    */
-  RequestBuilder<Req, Res> idempotencyKey(String idempotencyKey);
+  RequestBuilder<Req, Res> idempotencyKey(@Nullable String idempotencyKey);
 
   /**
    * @param idempotencyKey Idempotency key to attach in the request.
@@ -40,7 +40,7 @@ public interface RequestBuilder<Req, Res> extends Request<Req, Res> {
    * @param newHeaders headers to send together with the request.
    * @return this instance, so the builder can be used fluently.
    */
-  RequestBuilder<Req, Res> headers(Map<String, String> newHeaders);
+  RequestBuilder<Req, Res> headers(@Nullable Map<String, String> newHeaders);
 
   /**
    * @param headers headers to send together with the request. This will overwrite the already

common/src/main/java/dev/restate/common/RequestImpl.java

@@ -155,10 +155,12 @@ public Builder<Req, Res> header(String key, String value) {
      */
     @Override
     public Builder<Req, Res> headers(Map<String, String> newHeaders) {
-      if (this.headers == null) {
-        this.headers = new LinkedHashMap<>();
+      if (newHeaders != null) {
+        if (this.headers == null) {
+          this.headers = new LinkedHashMap<>();
+        }
+        this.headers.putAll(newHeaders);
       }
-      this.headers.putAll(newHeaders);
       return this;
     }