fix List.AllIndicesOf node (#13773)

Author: aparajit-pratap

src/Libraries/CoreNodes/List.cs

@@ -1,4 +1,4 @@
-using System;
+using System;
 using System.Collections;
 using System.Collections.Generic;
 using System.Linq;
@@ -1346,7 +1346,7 @@ public static IList AllIndicesOf(IList list, object item)
             if (list == null)
                 return new List<int> { }; 
 
-            var indices = Enumerable.Range(0, list.Count).Where(i => list[i].Equals(item)).ToList();
+            var indices = Enumerable.Range(0, list.Count).Where(i => list[i] != null ? list[i].Equals(item) : item == null).ToList();
             return indices;
         }
 

test/Libraries/CoreNodesTests/ListTests.cs

@@ -1,4 +1,4 @@
-using System;
+using System;
 using System.Collections;
 using System.Collections.Generic;
 using System.Linq;
@@ -948,6 +948,25 @@ public static void AllIndicesOf()
             Assert.IsEmpty(indices);
         }
 
+        [Test]
+        [Category("UnitTests")]
+        public static void AllIndicesOfNullTest()
+        {
+            var input = new List<object> { true, false, null };
+
+            var indices = List.AllIndicesOf(input, true);
+            Assert.True(indices.Count == 1);
+            Assert.AreEqual(0, indices[0]);
+
+            indices = List.AllIndicesOf(input, false);
+            Assert.True(indices.Count == 1);
+            Assert.AreEqual(1, indices[0]);
+
+            indices = List.AllIndicesOf(input, null);
+            Assert.True(indices.Count == 1);
+            Assert.AreEqual(2, indices[0]);
+        }
+
         [Test]
         [Category("UnitTests")]
         public static void CleanNullsPreserveIndices()