replace None with 'null' in query paramters

Requests skips parameters that are None.
Map 'null' string to None python type for a more native coding

pynetbox/core/endpoint.py

@@ -285,8 +285,9 @@ def filter(self, *args, **kwargs):
         offset = kwargs.pop("offset") if "offset" in kwargs else None
         if limit == 0 and offset is not None:
             raise ValueError("offset requires a positive limit value")
+        filters = {x: y if y is not None else "null" for x, y in kwargs.items()}
         req = Request(
-            filters=kwargs,
+            filters=filters,
             base=self.url,
             token=self.token,
             http_session=self.api.http_session,