Introduce sessionAttributeDoesNotExist in RequestResultMatchers

Analogous to the attributeDoesNotExist() method in ModelResultMatchers,
this commit introduces a new sessionAttributeDoesNotExist() method in
RequestResultMatchers which asserts that the given attributes are null
in the HttpSession.

Closes gh-23756

spring-test/src/main/java/org/springframework/test/web/servlet/result/RequestResultMatchers.java

@@ -31,6 +31,7 @@ import org.springframework.web.context.request.async.WebAsyncTask;
 
 import static org.hamcrest.MatcherAssert.assertThat;
 import static org.springframework.test.util.AssertionErrors.assertEquals;
+import static org.springframework.test.util.AssertionErrors.assertTrue;
 
 /**
  * Factory for assertions on the request.
@@ -150,6 +151,19 @@ public class RequestResultMatchers {
 		};
 	}
 
+	/**
+	 * Assert the given session attributes do not exist.
+	 */
+	public <T> ResultMatcher sessionAttributeDoesNotExist(String... names) {
+		return result -> {
+			HttpSession session = result.getRequest().getSession();
+			Assert.state(session != null, "No HttpSession");
+			for (String name : names) {
+				assertTrue("Session attribute '" + name + "' exists", session.getAttribute(name) == null);
+			}
+		};
+	}
+
 	private static void assertAsyncStarted(HttpServletRequest request) {
 		assertEquals("Async started", true, request.isAsyncStarted());
 	}