In stream_list, return 0 instead of None for rows and seconds

For rows and seconds only.  Extents still give None if they don't
exist.

nilmdb/server/nilmdb.py

@@ -290,8 +290,8 @@ class NilmDB(object):
         query = "SELECT streams.path, streams.layout"
         if extended:
             query += ", min(ranges.start_time), max(ranges.end_time) "
-            query += ", sum(ranges.end_pos - ranges.start_pos) "
-            query += ", sum(ranges.end_time - ranges.start_time) "
+            query += ", coalesce(sum(ranges.end_pos - ranges.start_pos), 0) "
+            query += ", coalesce(sum(ranges.end_time - ranges.start_time), 0) "
         query += " FROM streams"
         if extended:
             query += " LEFT JOIN ranges ON streams.id = ranges.stream_id"