feat(json): Improve mapper API

Author: PerfectSlayer

components/json/src/main/java/datadog/json/JsonMapper.java

@@ -1,5 +1,6 @@
 package datadog.json;
 
+import java.util.Collection;
 import java.util.Map;
 
 /** Utility class for simple Java structure mapping into JSON strings. */
@@ -68,8 +69,8 @@ public static String toJson(Map<String, ?> map) {
    * @return The converted JSON array as Java string.
    */
   @SuppressWarnings("DuplicatedCode")
-  public static String toJson(Iterable<String> items) {
-    if (items == null) {
+  public static String toJson(Collection<String> items) {
+    if (items == null || items.isEmpty()) {
       return "[]";
     }
     try (JsonWriter writer = new JsonWriter()) {

components/json/src/test/groovy/datadog/json/JsonMapperTest.groovy

@@ -32,7 +32,7 @@ class JsonMapperTest extends Specification {
 
   def "test mapping iterable to JSON array: #input"() {
     when:
-    String json = JsonMapper.toJson((Iterable) input)
+    String json = JsonMapper.toJson(input as Collection<String>)
 
     then:
     json == expected

dd-trace-core/src/main/java/datadog/trace/civisibility/writer/ddintake/CiTestCycleMapperV1.java

@@ -357,7 +357,7 @@ public void accept(Metadata metadata) {
           if (!(value instanceof Iterable)) {
             writable.writeObjectString(value, null);
           } else {
-            String serializedValue = toJson((Iterable<String>) value);
+            String serializedValue = toJson((Collection<String>) value);
             writable.writeString(serializedValue, null);
           }
         }