Build: check for self.data.version being None (#9430)

Ref https://sentry.io/organizations/read-the-docs/issues/3422620605/?alert_rule_id=229005&alert_timestamp=1657709758585&alert_type=email&environment=production&project=148442&referrer=alert_email

Author: stsewd

readthedocs/projects/tasks/builds.py

@@ -619,7 +619,8 @@ def after_return(self, status, retval, task_id, args, kwargs, einfo):
 
         build_complete.send(sender=Build, build=self.data.build)
 
-        clean_build(self.data.version)
+        if self.data.version:
+            clean_build(self.data.version)
 
         log.info(
             'Build finished.',