Do not send null HTTP header value in JdkClientHttpRequest

Prior to this commit, the `JdkClientHttpRequest` would add all values from `HttpHeaders` to the native request builder. This could cause `NullPointerException` being thrown at runtime because the `HttpClient` does not support that. This commit replicates a fix that was applied to the `SimpleClientHttpRequest`, turning null values into empty "". Fixes gh-35996

Author: bclozel

spring-web/src/main/java/org/springframework/http/client/JdkClientHttpRequest.java

@@ -175,7 +175,7 @@ private HttpRequest buildRequest(HttpHeaders headers, @Nullable Body body) {
 headers.forEach((headerName, headerValues) -> {
 if (!DISALLOWED_HEADERS.contains(headerName.toLowerCase(Locale.ROOT))) {
 for (String headerValue : headerValues) {
-builder.header(headerName, headerValue);
+builder.header(headerName, (headerValue != null) ? headerValue : "");
 }
 }
 });

spring-web/src/test/java/org/springframework/web/client/RestClientIntegrationTests.java

@@ -1024,6 +1024,29 @@ void defaultHeaders(ClientHttpRequestFactory requestFactory) throws IOException
 expectRequest(request -> assertThat(request.getHeaders().get("foo")).isEqualTo("bar"));
 }
 
+
+@ParameterizedRestClientTest
+void sendNullHeaderValue(ClientHttpRequestFactory requestFactory) throws IOException {
+startServer(requestFactory);
+
+prepareResponse(builder -> builder
+.setHeader("Content-Type", "text/plain").body("Hello Spring!"));
+
+String result = this.restClient.get()
+.uri("/greeting")
+.httpRequest(request -> request.getHeaders().add("X-Test-Header", null))
+.retrieve()
+.body(String.class);
+
+assertThat(result).isEqualTo("Hello Spring!");
+
+expectRequestCount(1);
+expectRequest(request -> {
+assertThat(request.getHeaders().get("X-Test-Header")).isNullOrEmpty();
+assertThat(request.getTarget()).isEqualTo("/greeting");
+});
+}
+
 @ParameterizedRestClientTest
 void defaultRequest(ClientHttpRequestFactory requestFactory) throws IOException {
 startServer(requestFactory);