Do not crash when no usage data for a gpu is available.

roles/gpu_detector/files/warn_gpu_not_used

@@ -126,6 +126,10 @@ def get_gpu_usage(node, start, end):
     for gpu in range(len(json.loads(data.content.decode())['data']['result'])):
         values += json.loads(data.content.decode())['data']['result'][gpu]['values']
 
+    if len(values) == 0:
+        # No values available.
+        return None
+
     average = sum([int(i[1]) for i in values]) / len(values)
     return average
 
@@ -215,13 +219,16 @@ def main():
     ])
     jobs = [line_to_job(l) for l in jobs.decode().split('\n') if l != '']
     jobs = list(filter(lambda a: a != None, jobs))
-    long_jobs = filter(lambda j: j.delta.total_seconds() > 3600, jobs)
+    long_jobs = filter(lambda j: j.delta.total_seconds() > 9000, jobs)
     message = []
     for job in long_jobs:
         gpus_usage = gpu_load(job)
         for entry in gpus_usage:
             gpu, usage = entry
-            job_info = f'Job id:  {job.id:10}  User: {job.user:9} Gpu usage: {usage:5.1f} ({gpu})'
+            if usage is None:
+                job_info = f'Job id:  {job.id:10}  User: {job.user:9} Gpu usage: N/A ({gpu})'
+            else:
+                job_info = f'Job id:  {job.id:10}  User: {job.user:9} Gpu usage: {usage:5.1f} ({gpu})'
             print(job_info)
 
             if usage == 0.0: