fix: use default group settings if not execution workflow (fixes attr…

…ibute error occurring with --report) (#2617)

This avoids the error `AttributeError: 'NoneType' object has no
attribute 'local_groupid'`, e.g. occurring when trying to create a
report

### QC
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* [ ] The PR contains a test case for the changes or the changes are
already covered by an existing test case.
* [ ] The documentation (`docs/`) is updated to reflect the changes or
this is not necessary (e.g. if the change does neither modify the
language nor the behavior or functionalities of Snakemake).

snakemake/api.py

@@ -379,6 +379,10 @@ def _workflow(self):
     def _get_workflow(self, **kwargs):
         from snakemake.workflow import Workflow
 
+        if "group_settings" not in kwargs:
+            # just init with defaults, can be overwritten later
+            kwargs["group_settings"] = GroupSettings()
+
         return Workflow(
             config_settings=self.config_settings,
             resource_settings=self.resource_settings,