Fix chained item function empty result comparison in conditions (#12901)

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: JanProvaznik <25267098+JanProvaznik@users.noreply.github.com>

Author: JanProvaznik

src/Build.UnitTests/Evaluation/Expander_Tests.cs

@@ -5344,5 +5344,35 @@ public void PropertyFunctionRegisterBuildCheck()
                 logger.AllBuildEvents.Count.ShouldBe(1);
             }
         }
+
+        /// <summary>
+        /// Test for issue where chained item functions with empty results incorrectly evaluate as non-empty in conditions
+        /// </summary>
+        [Fact]
+        public void ChainedItemFunctionEmptyResultInCondition()
+        {
+            string content = @"
+<Project>
+  <Target Name='Test'>
+    <ItemGroup>
+      <TestItem Include='Test1' Foo='Bar' />
+      <TestItem Include='Test2' />
+    </ItemGroup>
+
+    <!-- This should be empty because Test1 has Foo='Bar', not 'Baz' -->
+    <PropertyGroup Condition=""'@(TestItem->WithMetadataValue('Identity', 'Test1')->WithMetadataValue('Foo', 'Baz'))' == ''"">
+      <EmptyResult>TRUE</EmptyResult>
+    </PropertyGroup>
+
+    <Message Text='EmptyResult=$(EmptyResult)' Importance='high' />
+  </Target>
+</Project>
+            ";
+
+            MockLogger log = Helpers.BuildProjectWithNewOMExpectSuccess(content);
+
+            // The chained WithMetadataValue should return empty, so the condition should be true and EmptyResult should be set
+            log.AssertLogContains("EmptyResult=TRUE");
+        }
     }
 }

src/Build/Evaluation/Expander.cs

@@ -2065,21 +2065,25 @@ internal static List<KeyValuePair<string, S>> Transform<S>(
                             break;
                     }
 
+                    // If we have another transform, swap the source and transform lists.
+                    if (i < captures.Count - 1)
+                    {
+                        (transformedItems, sourceItems) = (sourceItems, transformedItems);
+                        transformedItems.Clear();
+                    }
+                }
+
+                // Check for break on non-empty only after ALL transforms are complete
+                if ((options & ExpanderOptions.BreakOnNotEmpty) != 0)
+                {
                     foreach (KeyValuePair<string, S> itemTuple in transformedItems)
                     {
-                        if (!string.IsNullOrEmpty(itemTuple.Key) && (options & ExpanderOptions.BreakOnNotEmpty) != 0)
+                        if (!string.IsNullOrEmpty(itemTuple.Key))
                         {
                             brokeEarly = true;
                             return transformedItems; // break out early
                         }
                     }
-
-                    // If we have another transform, swap the source and transform lists.
-                    if (i < captures.Count - 1)
-                    {
-                        (transformedItems, sourceItems) = (sourceItems, transformedItems);
-                        transformedItems.Clear();
-                    }
                 }
 
                 brokeEarly = false;