Fix the TypeError exception in the images.prune method

If the prune doesn't remove images,
the API returns "null" (with 200) and it's interpreted as
None (NoneType) so the for loop throws:
"TypeError: 'NoneType' object is not iterable".

My fix handles the above described case and the
client.images.prune() returns a valid Dict with zero
values, which is correct because the Podman didn't
remove anything.

Signed-off-by: Milan Balazs <milanbalazs01@gmail.com>

podman/domain/images_manager.py

@@ -158,17 +158,21 @@ def prune(
         deleted: List[Dict[str, str]] = []
         error: List[str] = []
         reclaimed: int = 0
-        for element in response.json():
-            if "Err" in element and element["Err"] is not None:
-                error.append(element["Err"])
-            else:
-                reclaimed += element["Size"]
-                deleted.append(
-                    {
-                        "Deleted": element["Id"],
-                        "Untagged": "",
-                    }
-                )
+        # If the prune doesn't remove images, the API returns "null" and it's interpreted as None (NoneType)
+        # so the for loop throws "TypeError: 'NoneType' object is not iterable".
+        # The below if condition fixes this issue.
+        if response.json() is not None:
+            for element in response.json():
+                if "Err" in element and element["Err"] is not None:
+                    error.append(element["Err"])
+                else:
+                    reclaimed += element["Size"]
+                    deleted.append(
+                        {
+                            "Deleted": element["Id"],
+                            "Untagged": "",
+                        }
+                    )
         if len(error) > 0:
             raise APIError(response.url, response=response, explanation="; ".join(error))
 

podman/tests/unit/test_imagesmanager.py

@@ -223,6 +223,18 @@ def test_prune_failure(self, mock):
             self.client.images.prune()
         self.assertEqual(e.exception.explanation, "Test prune failure in response body.")
 
+    @requests_mock.Mocker()
+    def test_prune_empty(self, mock):
+        """Unit test if prune API responses null (None)."""
+        mock.post(
+            tests.LIBPOD_URL + "/images/prune",
+            json=None
+        )
+
+        report = self.client.images.prune()
+        self.assertEqual(report["ImagesDeleted"], [])
+        self.assertEqual(report["SpaceReclaimed"], 0)
+
     @requests_mock.Mocker()
     def test_get(self, mock):
         mock.get(