Return correct values when running yarn in check mode (#153)

* Fixes Check Mode

* Adds parenthesis

* Adds changelog and tests

changelogs/fragments/153-yarn_fix_checkmode-ec61975fc65df7f0.yaml

@@ -0,0 +1,3 @@
+---
+bugfixes:
+- yarn - Return correct values when running yarn in check mode (https://github.com/ansible-collections/community.general/pull/153).

plugins/modules/packaging/language/yarn.py

@@ -218,7 +218,7 @@ class Yarn(object):
             rc, out, err = self.module.run_command(cmd, check_rc=check_rc, cwd=cwd)
             return out, err
 
-        return ''
+        return(None, None)
 
     def list(self):
         cmd = ['list', '--depth=0', '--json']

tests/integration/targets/yarn/tasks/run.yml

@@ -69,6 +69,24 @@
         that:
           - not (yarn_install is changed)
 
+    - name: 'Install all packages in check mode.'
+      yarn:
+        path: '{{ output_dir }}'
+        executable: '{{ yarn_bin_path }}/yarn'
+        state: present
+      environment:
+        PATH: '{{ node_bin_path }}:{{ ansible_env.PATH }}'
+      check_mode: true
+      register: yarn_install_check
+
+    - name: verify test yarn global installation in check mode
+      assert:
+        that:
+          - yarn_install_check.err is defined
+          - yarn_install_check.out is defined
+          - yarn_install_check.err is none
+          - yarn_install_check.out is none
+
     - name: 'Install package with explicit version (older version of package)'
       yarn:
         path: '{{ output_dir }}'