ref: avoid filtering on id=None (#75162)

this technically works at runtime, but django-stubs (imo correctly)
flags this as a mistake

<!-- Describe your PR here. -->

Author: asottile-sentry

src/sentry/api/endpoints/project_rules.py

@@ -236,9 +236,12 @@ def find_duplicate(self) -> Rule | None:
         """
         Determines whether specified rule already exists, and if it does, returns it.
         """
-        existing_rules = Rule.objects.exclude(id=self._rule_id).filter(
-            project__id=self._project_id, status=ObjectStatus.ACTIVE
-        )
+        if self._rule_id is None:
+            all_rules = Rule.objects.all()
+        else:
+            all_rules = Rule.objects.exclude(id=self._rule_id)
+
+        existing_rules = all_rules.filter(project__id=self._project_id, status=ObjectStatus.ACTIVE)
         for existing_rule in existing_rules:
             keys_checked = 0
             keys_matched = 0