Fix zoned executor metric when unzoned is allowed

An executor can have a zone configured and at the same time allow
unzoned jobs. In this case the executor was not counted for the zoned
executor metric (online/accepting).

Change-Id: Ib39947e3403d828b595cf2479e64789e049e63cc

zuul/executor/server.py

@@ -3157,6 +3157,10 @@ class ExecutorServer(BaseMergeServer):
         self.allow_unzoned = get_default(self.config, 'executor',
                                          'allow_unzoned', False)
 
+        # If this executor has no zone configured it is implicitly unzoned
+        if self.zone is None:
+            self.allow_unzoned = True
+
         # Those attributes won't change, so it's enough to set them once on the
         # component info.
         self.component_info.zone = self.zone