add safety mesasure for monitoring in case a service didn't find any results.

pyinfra/payload_processing/monitor.py

@@ -46,6 +46,9 @@ class PrometheusMonitor:
 
             runtime = time() - start
 
+            if not result:
+                return result
+
             processing_time_per_entity = runtime / len(result)
 
             self.entity_processing_time_sum.observe(processing_time_per_entity)