get_feeds() broken when there are too many feeds with no title

[lemon24]

(discovered during #196)

>>> from reader import make_reader
>>> reader = make_reader('')
>>> reader.add_feed('1')
>>> reader.add_feed('2')
>>> reader.add_feed('3')
>>> [f.url for f in reader.get_feeds()]
None
['1', '2', '3']
>>> reader._storage.chunk_size = 1
>>> [f.url for f in reader.get_feeds()]
None
['1']

This happens because the pagination adds the equivalent of WHERE kinda_title < :last_0; if any of those is null, the result is null.

reader/src/reader/_storage.py

Lines 475 to 477 in 30141c4

	if sort == 'title':
	query.SELECT(("kinda_title", "lower(coalesce(user_title, title))"))
	query.scrolling_window_order_by("kinda_title", "url")

The solution is likely to always coalesce to a non-null value (e.g. the empty string); we could either do it in the SELECT (if we know the value isn't used as a result), or globally in scrolling_window_order_by() (we need to see which SQLite type sorts lowest).

This is probably affecting other methods as well.