Fix error transaction name serialization check and add tests (elastic#2246)

Author: eyalkoren

apm-agent-core/src/main/java/co/elastic/apm/agent/report/serialize/DslJsonSerializer.java

@@ -339,9 +339,7 @@ private void serializeError(ErrorCapture errorCapture) {
     private void serializeErrorTransactionInfo(ErrorCapture.TransactionInfo errorTransactionInfo) {
         writeFieldName("transaction");
         jw.writeByte(JsonWriter.OBJECT_START);
-        if (errorTransactionInfo.getName() != null) {
-            writeField("name", errorTransactionInfo.getName());
-        }
+        writeField("name", errorTransactionInfo.getName());
         if (errorTransactionInfo.getType() != null) {
             writeField("type", errorTransactionInfo.getType());
         }

apm-agent-core/src/test/java/co/elastic/apm/agent/report/serialize/DslJsonSerializerTest.java

@@ -157,6 +157,7 @@ void testErrorSerialization() {
         ErrorCapture error = new ErrorCapture(tracer).asChildOf(transaction).withTimestamp(5000);
         error.setTransactionSampled(true);
         error.setTransactionType("test-type");
+        error.setTransactionName(new StringBuilder("Test Transaction"));
         error.setException(new Exception("test"));
         error.getContext().addLabel("foo", "bar");
 
@@ -176,8 +177,10 @@ void testErrorSerialization() {
         JsonNode stacktrace = exception.get("stacktrace");
         assertThat(stacktrace).hasSize(15);
 
-        assertThat(errorTree.get("transaction").get("sampled").booleanValue()).isTrue();
-        assertThat(errorTree.get("transaction").get("type").textValue()).isEqualTo("test-type");
+        JsonNode transactionTree = errorTree.get("transaction");
+        assertThat(transactionTree.get("sampled").booleanValue()).isTrue();
+        assertThat(transactionTree.get("type").textValue()).isEqualTo("test-type");
+        assertThat(transactionTree.get("name").asText()).isEqualTo("Test Transaction");
     }
 
     @Test
@@ -212,6 +215,11 @@ void testErrorSerializationWithEmptyTraceId() {
         assertThat(errorTree.get("trace_id")).isNull();
         assertThat(errorTree.get("parent_id")).isNull();
         assertThat(errorTree.get("transaction_id")).isNull();
+
+        JsonNode transactionTree = errorTree.get("transaction");
+        assertThat(transactionTree.get("sampled").booleanValue()).isFalse();
+        assertThat(transactionTree.get("type")).isNull();
+        assertThat(transactionTree.get("name")).isNull();
     }
 
     @Test