Store default of "None" as empty string for integer configuration items

Author: sde1000

quicktill/config.py

@@ -160,6 +160,8 @@ def from_db(cls, s):
 
     @classmethod
     def to_db(cls, v):
+        if v is None:
+            return ""
         return str(v)
 
 
@@ -181,6 +183,8 @@ def from_db(cls, s):
 
     @classmethod
     def to_db(cls, v):
+        if v is None:
+            return ""
         return str(v)