In the cloze deletion case, we already created a dummy card 0 when
no cloze deletions were found. This change makes normal note types
behave the same way - if no cards would be generated, a dummy card
0 is added to allow the note to be added.
This also applies when modifying note types - it is now possible
to delete card templates even if some notes only use that template,
as a dummy card 0 will be generated for notes that end up with no
cards left.
Still a prototype at this stage - we'll likely want a caching layer
for note types, and I'm not sure of the merit of having fields in
a separate table, since they're almost always required.
Searches that require multiple deck or note type lookups won't perform
very well at the moment - it either needs caching or to be split up
at the DB level.
Nothing tested yet.
