add a test for handling empty trailers

http2-tests/src/test/scala/org/apache/pekko/http/impl/engine/http2/HttpMessageRenderingSpec.scala

@@ -13,14 +13,14 @@
 
 package org.apache.pekko.http.impl.engine.http2
 
+import com.typesafe.config.ConfigFactory
 import java.time.format.DateTimeFormatter
 import org.apache.pekko
+import org.apache.pekko.http.scaladsl.settings.{ ClientConnectionSettings, ServerSettings }
 import pekko.event.NoLogging
 import pekko.http.impl.engine.rendering.DateHeaderRendering
 import pekko.http.scaladsl.model.headers._
-import pekko.http.scaladsl.model.{ ContentTypes, DateTime, HttpHeader, TransferEncodings }
-
-import scala.collection.immutable.Seq
+import pekko.http.scaladsl.model._
 import scala.collection.immutable.VectorBuilder
 import scala.util.Try
 import org.scalatest.matchers.should.Matchers
@@ -147,6 +147,22 @@ class HttpMessageRenderingSpec extends AnyWordSpec with Matchers {
       value1.exists(_._1 == "date") shouldBe false
     }
 
+    "handle empty trailer" in {
+      val config = ConfigFactory.load("reference.conf")
+      Try {
+        val rendering = new RequestRendering(ClientConnectionSettings(config), NoLogging)
+        rendering(HttpRequest().withAttributes(Map(AttributeKeys.trailer -> Trailer())))
+      }.isSuccess shouldBe true
+      Try {
+        val rendering = new ResponseRendering(ServerSettings(config), NoLogging, dateHeaderRendering)
+        rendering(
+          HttpResponse().withAttributes(
+            Map(
+              AttributeKeys.trailer -> Trailer(),
+              Http2.streamId -> 0)))
+      }.isSuccess shouldBe true
+    }
+
   }
 
   private def renderClientHeaders(headers: immutable.Seq[HttpHeader], builder: VectorBuilder[(String, String)],
@@ -158,10 +174,12 @@ class HttpMessageRenderingSpec extends AnyWordSpec with Matchers {
       peerIdHeader: Option[(String, String)] = None): Unit =
     HttpMessageRendering.renderHeaders(headers, builder, peerIdHeader, NoLogging, isServer = true,
       shouldRenderAutoHeaders = true,
-      dateHeaderRendering = new DateHeaderRendering {
-        // fake date rendering
-        override def renderHeaderPair(): (String, String) = "date" -> DateTime.now.toRfc1123DateTimeString
-        override def renderHeaderBytes(): Array[Byte] = ???
-        override def renderHeaderValue(): String = ???
-      })
+      dateHeaderRendering = dateHeaderRendering)
+
+  private lazy val dateHeaderRendering = new DateHeaderRendering {
+    // fake date rendering
+    override def renderHeaderPair(): (String, String) = "date" -> DateTime.now.toRfc1123DateTimeString
+    override def renderHeaderBytes(): Array[Byte] = ???
+    override def renderHeaderValue(): String = ???
+  }
 }