Fix mzml_portal for 'empty' scans

Author: RJMW

dimspy/portals/mzml_portal.py

@@ -71,7 +71,10 @@ def peaklist(self, scan_id, function_noise="median"):
         for scan in self.run():
             if scan["id"] == scan_id:
 
-                mzs, ints = zip(*scan.peaks)
+                if len(scan.peaks) > 0:
+                    mzs, ints = zip(*scan.peaks)
+                else:
+                    mss, ints = [], []
 
                 scan_time = scan["MS:1000016"]
                 tic = scan["total ion current"]