Make result=true if Docker volume already exists

In other states the result is always returned as `True` if no change is
required, whether `test=True` is set or not.

Fixes #42076

salt/states/docker_volume.py

@@ -187,7 +187,7 @@ def present(name, driver=None, driver_opts=None, force=False):
                 ret['result'] = result
                 return ret
 
-    ret['result'] = None if __opts__['test'] else True
+    ret['result'] = True
     ret['comment'] = 'Volume \'{0}\' already exists.'.format(name)
     return ret