Make it easier to apply a view's tagset as another view's filter

[fluffy-critter]

Currently, if you do something like get_view(category='blah', tag=view.tags) then if the current view has no tags, the view.tags will be [], which will cause no entries to match.

There are two possibilities for fixing this:

1. Make view.tags return None if the tagset is empty
2. Make get_view(tag=[]) apply no tag filter

Choice 2 makes the most high-level sense, but doing that would eliminate the possibility of purposefully excluding entries that do have tags, although it isn't clear if get_view(tag=[]) currently works at all anyway.

The current workaround is to do something like

get_view(tag=view.tags or None)

which also feels super grody.

[fluffy-critter]

Of course, it's better to derive views instead of generating them with get_view but sometimes that leads to other issues you have to work around instead.