Remove null rules from Categorizer

and allow for multiple null rules on the Nullifier.

Also create an empty list on the Nullifier constructor to simplify the
logic afterwards, in the case no "null" rule exist.

pfbudget/core/manager.py

@@ -101,10 +101,20 @@ class Manager:
                     categories = session.select(Category)
                     tags = session.select(Tag)
 
-                    rules = [cat.rules for cat in categories if cat.name == "null"]
+                    rules = [
+                        rule
+                        for cat in categories
+                        if cat.name == "null"
+                        for rule in cat.rules
+                    ]
                     Nullifier(rules).transform_inplace(uncategorized)
 
-                    rules = [rule for cat in categories for rule in cat.rules]
+                    rules = [
+                        rule
+                        for cat in categories
+                        if cat.name != "null"
+                        for rule in cat.rules
+                    ]
                     Categorizer(rules).transform_inplace(uncategorized)
 
                     rules = [rule for tag in tags for rule in tag.rules]

pfbudget/transform/nullifier.py

@@ -15,7 +15,7 @@ class Nullifier(Transformer):
     NULL_DAYS = 4
 
     def __init__(self, rules=None):
-        self.rules = rules
+        self.rules = rules if rules else []
 
     def transform(self, transactions: Sequence[Transaction]) -> Sequence[Transaction]:
         """transform