fix testServerCloseConnectionMidStream (#116792)

Author: mhl-b

modules/transport-netty4/src/internalClusterTest/java/org/elasticsearch/http/netty4/Netty4IncrementalRequestHandlingIT.java

@@ -210,7 +210,9 @@ public void testServerCloseConnectionMidStream() throws Exception {
             // terminate connection on server and wait resources are released
             handler.channel.request().getHttpChannel().close();
             assertBusy(() -> {
-                assertNull(handler.stream.buf());
+                // Cannot be simplified to assertNull.
+                // assertNull requires object to not fail on toString() method, but closing buffer can
+                assertTrue(handler.stream.buf() == null);
                 assertTrue(handler.streamClosed);
             });
         }

muted-tests.yml

@@ -232,9 +232,6 @@ tests:
   issue: https://github.com/elastic/elasticsearch/issues/116730
 - class: org.elasticsearch.xpack.security.authc.ldap.ActiveDirectoryGroupsResolverTests
   issue: https://github.com/elastic/elasticsearch/issues/116182
-- class: org.elasticsearch.http.netty4.Netty4IncrementalRequestHandlingIT
-  method: testServerCloseConnectionMidStream
-  issue: https://github.com/elastic/elasticsearch/issues/116774
 - class: org.elasticsearch.xpack.test.rest.XPackRestIT
   method: test {p0=snapshot/20_operator_privileges_disabled/Operator only settings can be set and restored by non-operator user when operator privileges is disabled}
   issue: https://github.com/elastic/elasticsearch/issues/116775