make the user2uid/group2gid win32 hack behave more like the real code (e.g. posix)

The code relies on them returning the default value (usually
None), if we call user2uid(None) or group2gid(None) (same for
empty string).

src/borg/platform/windows.pyx

@@ -21,6 +21,9 @@ def uid2user(uid, default=None):
 
 @lru_cache(maxsize=None)
 def user2uid(user, default=None):
+    if not user:
+        # user is either None or the empty string
+        return default
     return 0
 
 
@@ -31,6 +34,9 @@ def gid2group(gid, default=None):
 
 @lru_cache(maxsize=None)
 def group2gid(group, default=None):
+    if not group:
+        # group is either None or the empty string
+        return default
     return 0