[edftomoscan] guess_index: return if part_1 is None

tomoscan/esrf/edfscan.py

@@ -327,6 +327,8 @@ class EDFTomoScan(TomoScanBase):
             else:
                 part_2 = None
 
+            if part_1 is None:
+                return None
             if part_2 is None:
                 return int(part_1)
             else: