Allow empty foldernames

Empty foldernames (as they could be created through nametrans) were
failing as the uidvalidity and status files names as determined by
folder/Base.py:getfolderbasename() lead to invalid file names ''.

Fix this by handling empty file names and translating them to '.' which
leads to the special file name 'dot'. (this special value existed before
and was not invented by this patch)

Signed-off-by: Sebastian Spaeth <Sebastian@SSpaeth.de>
Signed-off-by: Nicolas Sebrecht <nicolas.s-dev@laposte.net>

offlineimap/folder/LocalStatusSQLite.py

@@ -115,7 +115,7 @@ class LocalStatusSQLiteFolder(LocalStatusFolder):
             plaintextfilename = os.path.join(
                 self.repository.account.getaccountmeta(),
                 'LocalStatus',
-                re.sub('(^|\/)\.$','\\1dot', self.name))
+                self.getfolderbasename(self.name))
             # MIGRATE from plaintext if needed
             if os.path.exists(plaintextfilename):
                 self.ui._msg('Migrating LocalStatus cache from plain text '