only add data when there is data to add (#54559)

Only add data when there is data to add

  also avoid clobbering existing data with empty file
  fixes #45843

* remove redundant code, update comments
* fix mock dataloader, original does not return None
* added test

test/integration/targets/roles/data_integrity.yml

@@ -0,0 +1,4 @@
+- hosts: all
+  gather_facts: false
+  roles:
+    - data

test/integration/targets/roles/roles/data/defaults/main/00.yml

@@ -0,0 +1 @@
+defined_var: 1

test/integration/targets/roles/roles/data/tasks/main.yml

@@ -0,0 +1,5 @@
+- name: ensure data was correctly defind
+  assert:
+    that:
+      - defined_var is defined
+      - defined_var == 1

test/integration/targets/roles/runme.sh

@@ -12,3 +12,7 @@ set -eux
 # include/import can execute another instance of role
 [ "$(ansible-playbook allowed_dupes.yml -i ../../inventory --tags importrole "$@" | grep -c '"msg": "A"')" = "2" ]
 [ "$(ansible-playbook allowed_dupes.yml -i ../../inventory --tags includerole "$@" | grep -c '"msg": "A"')" = "2" ]
+
+
+# ensure role data is merged correctly
+ansible-playbook data_integrity.yml -i ../../inventory "$@"

test/units/mock/loader.py

@@ -39,10 +39,11 @@ class DictDataLoader(DataLoader):
         self._vault_secrets = None
 
     def load_from_file(self, path, cache=True, unsafe=False):
+        data = None
         path = to_text(path)
         if path in self._file_mapping:
-            return self.load(self._file_mapping[path], path)
-        return None
+            data = self.load(self._file_mapping[path], path)
+        return data
 
     # TODO: the real _get_file_contents returns a bytestring, so we actually convert the
     #       unicode/text it's created with to utf-8