fix: Initialize assignments dictionary when setting rule-based resour…

…ces (#1154)

* Fix issue #1080

Initialize assignments[rule] dictionary

* Use defaultdict to initialize assignments

* Add space after import

snakemake/__init__.py

@@ -817,7 +817,9 @@ def parse_set_resources(args):
         "VALUE being a positive integer or a string."
     )
 
-    assignments = dict()
+    from collections import defaultdict
+
+    assignments = defaultdict(dict)
     if args.set_resources is not None:
         for entry in args.set_resources:
             key, value = parse_key_value_arg(entry, errmsg=errmsg)