BF: define var (to None) before it might be assigned

Ran into

   UnboundLocalError: local variable ver referenced before assignment

although unclear how/why now since prior run (with a little bit different way to it)
did succeed for that matrix run with LC_ALL set to ru_RU.UTF-8

datalad/support/external_versions.py

@@ -163,6 +163,7 @@ def get_rsync_version():
         on_linux,
     )
     if on_linux:
+        ver = None
         dist = get_linux_distribution()[0]
         if dist in ['debian', 'ubuntu']:
             out = _runner.run(['apt-cache', 'policy', 'rsync'],