Fix handling of use_known_patches=False

- used to cause an exception

Author: mrbean-bremen

CHANGES.md

@@ -12,6 +12,7 @@ The released versions correspond to PyPi releases.
   (see [#627](../../issues/627))
 * handle `pathlib.Path.owner()` and `pathlib.Path.group` by returning 
   the current user/group name (see [#629](../../issues/629))
+* fixed handling of `use_known_patches=False` (could cause an exception)  
 
 ### Infrastructure
 * added test dependency check (see [#608](../../issues/608))

pyfakefs/fake_filesystem_unittest.py

@@ -485,6 +485,8 @@ def __init__(self, additional_skip_names: Optional[
             for name, fake_module in modules_to_patch.items():
                 self._fake_module_classes[name] = fake_module
             patched_module_names = set(modules_to_patch)
+        else:
+            patched_module_names = set()
         clear_cache = not use_cache
         if use_cache:
             if patched_module_names != self.PATCHED_MODULE_NAMES: