Allow null from RestClient exchange methods

Closes gh-33779

spring-web/src/main/java/org/springframework/web/client/DefaultRestClient.java

@@ -455,14 +455,18 @@ final class DefaultRestClient implements RestClient {
 
 		@Override
 		public ResponseSpec retrieve() {
-			return exchangeInternal(DefaultResponseSpec::new, false);
+			ResponseSpec responseSpec = exchangeInternal(DefaultResponseSpec::new, false);
+			Assert.state(responseSpec != null, "No ResponseSpec");
+			return responseSpec;
 		}
 
 		@Override
+		@Nullable
 		public <T> T exchange(ExchangeFunction<T> exchangeFunction, boolean close) {
 			return exchangeInternal(exchangeFunction, close);
 		}
 
+		@Nullable
 		private <T> T exchangeInternal(ExchangeFunction<T> exchangeFunction, boolean close) {
 			Assert.notNull(exchangeFunction, "ExchangeFunction must not be null");
 

spring-web/src/main/java/org/springframework/web/client/RestClient.java

@@ -561,6 +561,7 @@ public interface RestClient {
 		 * @param <T> the type the response will be transformed to
 		 * @return the value returned from the exchange function
 		 */
+		@Nullable
 		default <T> T exchange(ExchangeFunction<T> exchangeFunction) {
 			return exchange(exchangeFunction, true);
 		}
@@ -592,6 +593,7 @@ public interface RestClient {
 		 * @param <T> the type the response will be transformed to
 		 * @return the value returned from the exchange function
 		 */
+		@Nullable
 		<T> T exchange(ExchangeFunction<T> exchangeFunction, boolean close);
 
 
@@ -609,6 +611,7 @@ public interface RestClient {
 			 * @return the exchanged type
 			 * @throws IOException in case of I/O errors
 			 */
+			@Nullable
 			T exchange(HttpRequest clientRequest, ConvertibleClientHttpResponse clientResponse) throws IOException;
 		}