Fix missing call to str.strip() in get_next_reference (#12966)

The missing call would throw a TypeError, but that was later on swallowed, so the code would fall back to `reference_int` and never used the configured reference pattern

Co-authored-by: Matthias Mair <code@mjmair.com>
Co-authored-by: Oliver <oliver.henry.walters@gmail.com>

src/backend/InvenTree/InvenTree/models.py

@@ -332,7 +332,7 @@ class ReferenceIndexingMixin(models.Model):
             # No existing items
             return 1
 
-        reference = latest.reference.strip
+        reference = latest.reference.strip()
 
         try:
             reference = InvenTree.format.extract_named_group(