Fix publish action on mock backend

Already published products got overwritten

Author: tleguijt

CHANGES

@@ -1,3 +1,7 @@
+8.1.1 (unreleased)
+------------------
+- Fix publish action on mock backend; already published products got overwritten
+
 8.1.0 (2020-05-01)
 ------------------
 - Added product publish and set prices actions on mock backend.

src/commercetools/testing/products.py

@@ -231,7 +231,7 @@ def _publish_product_action():
     def updater(self, obj: dict, action: types.ProductPublishAction):
         new = copy.deepcopy(obj)
         # not implemented scopes right now.
-        if "staged" in new["masterData"]:
+        if new["masterData"].get("staged"):
             new["masterData"]["current"] = new["masterData"]["staged"]
         new["masterData"]["hasStagedChanges"] = False
         new["masterData"]["published"] = True