allow empty strings to be written to protobuf

Author: breedx-splk

exporters/common/src/main/java/io/opentelemetry/exporter/internal/marshal/Serializer.java

@@ -254,7 +254,11 @@ public void serializeRepeatedString(ProtoFieldInfo field, byte[][] utf8Bytes) th
    */
   public void serializeStringWithContext(
       ProtoFieldInfo field, @Nullable String string, MarshalerContext context) throws IOException {
-    if (string == null || string.isEmpty()) {
+    if (string == null) {
+      return;
+    }
+    if (string.isEmpty()) {
+      writeString(field, string, 0, context);
       return;
     }
     if (context.marshalStringNoAllocation()) {

exporters/common/src/test/java/io/opentelemetry/exporter/internal/marshal/ProtoSerializerTest.java

@@ -0,0 +1,52 @@
+package io.opentelemetry.exporter.internal.marshal;
+
+import static java.nio.charset.StandardCharsets.UTF_8;
+import static org.assertj.core.api.AssertionsForClassTypes.assertThat;
+
+import java.io.ByteArrayOutputStream;
+import org.junit.jupiter.api.Test;
+
+class ProtoSerializerTest {
+
+  @Test
+  void testSerializeStringWithContext() throws Exception {
+    String value = "mystring";
+    ByteArrayOutputStream out = new ByteArrayOutputStream();
+
+    ProtoSerializer serializer = new ProtoSerializer(out);
+
+    ProtoFieldInfo field = ProtoFieldInfo.create(1, 10, "stringValue");
+    MarshalerContext context = new MarshalerContext();
+    context.setSize(0, value.length());
+    serializer.serializeStringWithContext(field, value, context);
+    serializer.close();
+
+    byte[] result = out.toByteArray();
+    assertThat(result.length).isEqualTo(10); // one byte tag, one byte length, rest string
+    assertThat((int)result[0]).isEqualTo(field.getTag());
+    assertThat((int)result[1]).isEqualTo(value.length());
+    assertThat(new String(result, 2, result.length-2, UTF_8)).isEqualTo(value);
+  }
+
+  @Test
+  void testSerializeEmptyStringWithContext() throws Exception {
+    String value = "";
+    ByteArrayOutputStream out = new ByteArrayOutputStream();
+
+    ProtoSerializer serializer = new ProtoSerializer(out);
+
+    ProtoFieldInfo field = ProtoFieldInfo.create(1, 10, "stringValue");
+    MarshalerContext context = new MarshalerContext();
+    context.setSize(0, 0);
+
+    serializer.serializeStringWithContext(field, value, context);
+    serializer.close();
+
+    byte[] result = out.toByteArray();
+
+    assertThat(result.length).isEqualTo(2);
+    assertThat((int)result[0]).isEqualTo(field.getTag());
+    assertThat((int)result[1]).isEqualTo(0);
+  }
+
+}