Use IntrospectingClientHttpResponse in RestClient

This commit ensures that the RestClient uses the
IntrospectingClientHttpResponse to verify whether the response has a
body, and return null if it does not.

See gh-12671
Closes gh-31719

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

@@ -348,6 +348,23 @@ class RestClientIntegrationTests {
 		assertThat(result).isNull();
 	}
 
+	@ParameterizedRestClientTest
+	@SuppressWarnings({ "rawtypes", "unchecked" })
+	void retrieveJsonEmpty(ClientHttpRequestFactory requestFactory) {
+		startServer(requestFactory);
+
+		prepareResponse(response -> response
+				.setResponseCode(200)
+				.setHeader(HttpHeaders.CONTENT_TYPE, MediaType.APPLICATION_JSON_VALUE));
+
+		Pojo result = this.restClient.get()
+				.uri("/null")
+				.retrieve()
+				.body(Pojo.class);
+
+		assertThat(result).isNull();
+	}
+
 	@ParameterizedRestClientTest
 	void retrieve404(ClientHttpRequestFactory requestFactory) {
 		startServer(requestFactory);