fix(testing): Fix JSON span link decoding

Author: PerfectSlayer

utils/test-agent-utils/decoder/src/main/java/datadog/trace/test/agent/decoder/json/raw/SpanJson.java

@@ -132,7 +132,7 @@ public List<DecodedSpanLink> getLinks() {
    * submitted a {@code v1.0} payload) and from the meta tag otherwise.
    */
   void resolveLinks() {
-    if (this.spanLinks == null || this.spanLinks.isEmpty()) {
+    if (this.spanLinks == null) {
       this.links = DecodedSpanLinks.fromMeta(getMeta());
       return;
     }

utils/test-agent-utils/decoder/src/test/java/datadog/trace/test/agent/decoder/SpanLinksDecoderTest.java

@@ -202,6 +202,23 @@ void structuredSpanLinksFieldWinsOverTheMetaTag() {
     assertEquals(7L, span.getLinks().get(0).getSpanId(), "the structured field is preferred");
   }
 
+  @Test
+  void anEmptyStructuredSpanLinksFieldWinsOverTheMetaTag() {
+    // A v1.0 payload always carries the span_links field, so an empty one means the span has no
+    // link, whichever legacy meta tag the payload also holds.
+    String json =
+        "[[{\"service\":\"s\",\"name\":\"n\",\"resource\":\"r\","
+            + "\"trace_id\":1,\"span_id\":1,\"parent_id\":0,\"start\":0,\"duration\":1,\"error\":0,"
+            + "\"metrics\":{},\"meta\":{\""
+            + SPAN_LINKS_TAG
+            + "\":\"[{\\\"trace_id\\\":\\\"1\\\",\\\"span_id\\\":\\\"99\\\"}]\"},"
+            + "\"span_links\":[]}]]";
+
+    DecodedSpan span = Decoder.decodeJson(json).getTraces().get(0).getSpans().get(0);
+
+    assertTrue(span.getLinks().isEmpty(), "the structured field is preferred, even when empty");
+  }
+
   @Test
   void decodedSpanWithoutTheTagExposesNoLink() {
     String json =