Use default if gearman-plugin None for node_labels

The default for node_labels is expected to be empty list not None.

Change-Id: I60ae82b5ef86aab5e20e91e25302c42390d2ac17
diff --git a/zuul/launcher/gearman.py b/zuul/launcher/gearman.py
index e43e867..69fb71b 100644
--- a/zuul/launcher/gearman.py
+++ b/zuul/launcher/gearman.py
@@ -424,7 +424,7 @@
         build = self.builds.get(job.unique)
         if build:
             data = getJobData(job)
-            build.node_labels = data.get('node_labels')
+            build.node_labels = data.get('node_labels', [])
             build.node_name = data.get('node_name')
             if not build.canceled:
                 if result is None: