fix(core): preserve explicit null changes

teaql-core/src/main/java/io/teaql/core/ChangeSetStack.java

@@ -31,8 +31,10 @@ public EntityChangeSet pop() {
 
     public Object get(EntityKey key, String field) {
         for (int i = stack.size() - 1; i >= 0; i--) {
-            Object value = stack.get(i).get(key, field);
-            if (value != null) return value;
+            EntityChangeSet changeSet = stack.get(i);
+            if (changeSet.contains(key, field)) {
+                return changeSet.get(key, field);
+            }
         }
         return null;
     }

teaql-core/src/main/java/io/teaql/core/EntityChangeSet.java

@@ -22,6 +22,11 @@ public Object get(EntityKey key, String field) {
         return record != null ? record.get(field) : null;
     }
 
+    boolean contains(EntityKey key, String field) {
+        Map<String, Object> record = changes.get(key);
+        return record != null && record.containsKey(field);
+    }
+
     public Map<EntityKey, Map<String, Object>> changes() {
         return Collections.unmodifiableMap(changes);
     }

teaql-core/src/test/java/io/teaql/core/ChangeSetStackTest.java

@@ -48,6 +48,21 @@ public void topScopeShadowsLowerScopeAndPopRestoresIt() {
         assertEquals("CREATED", stack.get(ORDER, "status"));
     }
 
+    @Test
+    public void explicitNullInTopScopeShadowsLowerValue() {
+        ChangeSetStack stack = new ChangeSetStack();
+        stack.set(ORDER, "status", "CREATED");
+        stack.push();
+        stack.set(ORDER, "status", null);
+
+        assertNull(stack.get(ORDER, "status"));
+
+        stack.pop();
+
+        assertEquals("CREATED", stack.get(ORDER, "status"));
+        assertNull(stack.get(ORDER, "missing"));
+    }
+
     @Test
     public void clearCurrentClearsOnlyTopScope() {
         ChangeSetStack stack = new ChangeSetStack();