Fix cookie management in reactive JdkClientHttpRequest

Prevent cookies to be set to empty in the HTTP request if no cookie
is present

See gh-32799

Author: Matteo Cristoforo

Diff for: spring-web/src/main/java/org/springframework/http/client/reactive/JdkClientHttpRequest.java

@@ -37,6 +37,7 @@
 import org.springframework.http.HttpHeaders;
 import org.springframework.http.HttpMethod;
 import org.springframework.util.Assert;
+import org.springframework.util.MultiValueMap;
 
 /**
  * {@link ClientHttpRequest} for the Java {@link HttpClient}.
@@ -108,8 +109,11 @@ protected void applyHeaders() {
 
 	@Override
 	protected void applyCookies() {
-		this.builder.header(HttpHeaders.COOKIE, getCookies().values().stream()
-				.flatMap(List::stream).map(HttpCookie::toString).collect(Collectors.joining(";")));
+		MultiValueMap<String, HttpCookie> cookies = getCookies();
+		if (!cookies.isEmpty()) {
+			this.builder.header(HttpHeaders.COOKIE, cookies.values().stream()
+					.flatMap(List::stream).map(HttpCookie::toString).collect(Collectors.joining(";")));
+		}
 	}
 
 	@Override