Update evaluation reason for empty waterfall to DEFAULT instead of ERROR/GENERAL per REASON.9 (#11193)

Fix REASON-9: empty allocations waterfall returns DEFAULT not GENERAL

fix(ffe): add null-allocations guard; fix empty-allocs test expectation

- DDEvaluator: add explicit null check for flag.allocations before the
  iteration loop. A null list throws NPE which gets swallowed as GENERAL;
  returning GENERAL explicitly with a clear message is safer and testable.
- DDEvaluatorTest.testNoAllocations: the empty-allocation case already
  falls through the for-loop to the DEFAULT return; the test was asserting
  ERROR/GENERAL which contradicts the fix. Update to assert DEFAULT with no
  error code.

Update error message for null allocations in feature flag

Co-authored-by: devflow.devflow-routing-intake <devflow.devflow-routing-intake@kubernetes.us1.ddbuild.io>

Author: typotter

products/feature-flagging/feature-flagging-api/src/main/java/datadog/trace/api/openfeature/DDEvaluator.java

@@ -101,8 +101,8 @@ public <T> ProviderEvaluation<T> evaluate(
             .build();
       }
 
-      if (isEmpty(flag.allocations)) {
-        return error(defaultValue, ErrorCode.GENERAL, "Missing allocations for flag " + flag.key);
+      if (flag.allocations == null) {
+        return error(defaultValue, ErrorCode.GENERAL, "Missing allocations for flag " + key);
       }
 
       final Date now = new Date();

products/feature-flagging/feature-flagging-api/src/test/java/datadog/trace/api/openfeature/DDEvaluatorTest.java

@@ -13,6 +13,7 @@
 import static java.util.Collections.emptyMap;
 import static java.util.Collections.singletonList;
 import static org.hamcrest.CoreMatchers.equalTo;
+import static org.hamcrest.CoreMatchers.nullValue;
 import static org.hamcrest.MatcherAssert.assertThat;
 import static org.hamcrest.Matchers.hasEntry;
 import static org.hamcrest.Matchers.oneOf;
@@ -174,8 +175,8 @@ public void testNoAllocations() {
 
     details = evaluator.evaluate(Integer.class, "empty-allocation", 23, ctx);
     assertThat(details.getValue(), equalTo(23));
-    assertThat(details.getReason(), equalTo(ERROR.name()));
-    assertThat(details.getErrorCode(), equalTo(ErrorCode.GENERAL));
+    assertThat(details.getReason(), equalTo(DEFAULT.name()));
+    assertThat(details.getErrorCode(), nullValue());
   }
 
   private static Arguments[] flatteningTestCases() {