alternatives: Fix bug with priority default (#4810)

* alternatives: Fix bug with priority default

If neigther the priority nor the subcommands where specified the module decided to update the priority with the default value anyway. This resulted in bug #4803 and #4804

* Add changelog fragment.

* Distinguish None from 0.

* Address review comments.

* Update plugins/modules/system/alternatives.py

Co-authored-by: Pilou <pierre-louis@libregerbil.fr>

* Remove unrelated issues from changelog.

Co-authored-by: Felix Fontein <felix@fontein.de>
Co-authored-by: Pilou <pierre-louis@libregerbil.fr>

tests/integration/targets/alternatives/tasks/test.yml

@@ -48,4 +48,4 @@
   when: ansible_os_family == 'RedHat' and not with_alternatives and item == 1
 
 - name: check that alternative has been updated
-  command: "grep -Pzq '/bin/dummy{{ item }}\\n50' '{{ alternatives_dir }}/dummy'"
+  command: "grep -Pzq '/bin/dummy{{ item }}\\n' '{{ alternatives_dir }}/dummy'"

tests/integration/targets/alternatives/tasks/tests_set_priority.yml

@@ -31,4 +31,19 @@
   register: alternative
 
 - name: check that alternative priority has been updated
-  command: "grep -Pzq '/bin/dummy{{ item }}\\n{{ 70 + item|int }}' '{{ alternatives_dir }}/dummy'"
+  command: "grep -Pzq '/bin/dummy{{ item }}\\n{{ 70 + item|int }}' '{{ alternatives_dir }}/dummy'"
+
+- name: no change without priority
+  alternatives:
+    name: dummy
+    path: '/usr/bin/dummy{{ item }}'
+    link: /usr/bin/dummy
+  register: alternative
+
+- name: check no change was triggered without priority
+  assert:
+    that:
+      - 'alternative is not changed'
+
+- name: check that alternative priority has not been changed
+  command: "grep -Pzq '/bin/dummy{{ item }}\\n{{ 70 + item|int }}' '{{ alternatives_dir }}/dummy'"