add comments and address feedback on tests from lukasz

Author: SiyaoIsHiding

core/src/main/java/com/datastax/oss/driver/api/core/tracker/RequestIdGenerator.java

@@ -75,6 +75,8 @@ default Statement<?> getDecoratedStatement(
     // Add or overwrite
     existing.put(key, ByteBuffer.wrap(requestId.getBytes(StandardCharsets.UTF_8)));
 
+    // Allowing null key/values
+    // Wrap a map inside to be immutable without instanciating a new map
     Map<String, ByteBuffer> unmodifiableMap = Collections.unmodifiableMap(existing);
 
     return statement.setCustomPayload(unmodifiableMap);

integration-tests/src/test/java/com/datastax/oss/driver/core/tracker/RequestIdGeneratorIT.java

@@ -121,7 +121,7 @@ public void should_not_write_id_to_custom_payload_when_key_is_not_set() {
     try (CqlSession session = SessionUtils.newSession(ccmRule, loader)) {
       String query = "SELECT * FROM system.local";
       ResultSet rs = session.execute(query);
-      assertThat(rs.getExecutionInfo().getRequest().getCustomPayload().get("trace_key")).isNull();
+      assertThat(rs.getExecutionInfo().getRequest().getCustomPayload().get("request-id")).isNull();
     }
   }
 
@@ -139,8 +139,6 @@ public void should_succeed_with_null_value_in_custom_payload() {
       SimpleStatement statement =
           SimpleStatement.newInstance(query).setCustomPayload(customPayload);
       assertThatStage(session.executeAsync(statement)).isSuccess();
-      ResultSet rs = session.execute(query);
-      assertThat(rs.getExecutionInfo().getRequest().getCustomPayload().get("trace_key")).isNull();
     }
   }
 }