bugfix for attribute error during bootstrap
initial bootstrap by attaching Patroni to the running postgres was causing the following error.
```
File "/xx/lib/python3.8/site-packages/patroni/ha.py", line 529, in update_cluster_history
history = history[-self.cluster.config.max_timelines_history:]
AttributeError: 'NoneType' object has no attribute 'max_timelines_history'
```
@@ -525,7 +525,7 @@ class Ha(object):
elif not cluster_history or cluster_history[-1][0] != master_timeline - 1 or len(cluster_history[-1]) != 4:
cluster_history = {line[0]: line for line in cluster_history or []}
history = self.state_handler.get_history(master_timeline)
- if history:
+ if history and self.cluster.config:
history = history[-self.cluster.config.max_timelines_history:]
for line in history:
# enrich current history with promotion timestamps stored in DCS
