Fix uninitialized distribution fact on FreeBSD (#15842)

Initialize facts['distribution'] with self.system so that this fact does
not remain uninitialized on systems_platform_working platforms (FreeBSD,
OpenBSD).

Fixes #15841

lib/ansible/module_utils/facts.py

@@ -660,6 +660,7 @@ class Distribution(object):
         # The platform module provides information about the running
         # system/distribution. Use this as a baseline and fix buggy systems
         # afterwards
+        self.facts['distribution'] = self.system
         self.facts['distribution_release'] = platform.release()
         self.facts['distribution_version'] = platform.version()