fix: bugfix to avoid reprocessing when no data update available

bec_plugins/data_processing/px_example.py

@@ -143,12 +143,14 @@ class StreamProcessorPx(StreamProcessor):
             # TODO debug code  for timing
             #
             data_msgs = self._get_data(proj_nr)
+            if data_msgs is None:
+                continue
             data.extend([msg.content["signals"]["data"] for msg in data_msgs if msg is not None])
             # print(f"Loading took {time.time() - start}")
             start = time.time()
             result = self.process(data, metadata)
             print(f"Processing took {time.time() - start}")
-            if not result:
+            if result is None:
                 continue
             print(f"Length of data is {result[0][0]['z'].shape}")
             msg = BECMessage.ProcessedDataMessage(data=result[0][0], metadata=result[1]).dumps()
@@ -170,7 +172,7 @@ class StreamProcessorPx(StreamProcessor):
             f"px_stream/projection_{proj_nr}/data", self.num_received_msgs, -1
         )
         if not msgs:
-            return []
+            return None
         self.num_received_msgs += len(msgs)
         return [BECMessage.DeviceMessage.loads(msg) for msg in msgs]