Handle build_set being None for priority

There are flows where there is no build_set. In those cases, return
a normal priority.

Also fixes a pep8 error from a force-merge.

Change-Id: I1c98508fe227f7f6aa1d0b2f7dcf270cecaa60d8
diff --git a/zuul/model.py b/zuul/model.py
index eff0ae3..6fe3b94 100644
--- a/zuul/model.py
+++ b/zuul/model.py
@@ -532,7 +532,11 @@
 
     @property
     def priority(self):
-        return PRIORITY_MAP[self.build_set.item.pipeline.precedence]
+        if self.build_set:
+            precedence = self.build_set.item.pipeline.precedence
+        else:
+            precedence = PRECEDENCE_NORMAL
+        return PRIORITY_MAP[precedence]
 
     @property
     def fulfilled(self):