LocalStatusSQLite: labels: don't fail if database returns unexpected None value

This requires more researches. See
  https://github.com/OfflineIMAP/offlineimap/issues/103
.

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

offlineimap/folder/LocalStatusSQLite.py

@@ -203,7 +203,21 @@ class LocalStatusSQLiteFolder(BaseFolder):
             uid = row[0]
             self.messagelist[uid] = self.msglist_item_initializer(uid)
             flags = set(row[1])
-            labels = set([lb.strip() for lb in row[3].split(',') if len(lb.strip()) > 0])
+            try:
+                labels = set([lb.strip() for lb in
+                    row[3].split(',') if len(lb.strip()) > 0])
+            except AttributeError:
+                # FIXME: This except clause was introduced because row[3] from
+                # database can be found of unexpected type NoneType. See
+                # https://github.com/OfflineIMAP/offlineimap/issues/103
+                #
+                # We are fixing the type here but this would require more
+                # researches to find the true root cause. row[3] is expected to
+                # be a (empty) string, not None.
+                #
+                # Also, since database might return None, we have to fix the
+                # database, too.
+                labels = set()
             self.messagelist[uid]['flags'] = flags
             self.messagelist[uid]['labels'] = labels
             self.messagelist[uid]['mtime'] = row[2]

offlineimap/folder/UIDMaps.py

@@ -208,7 +208,7 @@ class MappedIMAPFolder(IMAPFolder):
         if uid < 0:
             return uid
 
-        #if msg uid already exists, just modify the flags
+        # If msg uid already exists, just modify the flags.
         if uid in self.r2l:
             self.savemessageflags(uid, flags)
             return uid
@@ -238,11 +238,10 @@ class MappedIMAPFolder(IMAPFolder):
 
     # Interface from BaseFolder
     def savemessageflags(self, uid, flags):
-        """
-
-        Note that this function does not check against dryrun settings,
+        """Note that this function does not check against dryrun settings,
         so you need to ensure that it is never called in a
         dryrun mode."""
+
         self._mb.savemessageflags(self.r2l[uid], flags)
 
     # Interface from BaseFolder