Fix validation of duplicate IPN

- Duplicate IPN check does not apply if an empty IPN value is set
- Note that "if x" is a more pythonic test than "if x not in [None, '']"

InvenTree/stock/models.py

@@ -453,10 +453,12 @@ class StockItem(MPTTModel):
 
         super().clean()
 
-        if self.serial is not None and type(self.serial) is str:
+        # Strip serial number field
+        if self.serial:
             self.serial = self.serial.strip()
 
-        if self.batch is not None and type(self.batch) is str:
+        # Strip batch code field
+        if self.batch:
             self.batch = self.batch.strip()
 
         try: