As noticed in #88, multiple imports of the same data was leading to an
increase in entries in the `renovate` table, which shouldn't have been
possible.

The entries that are duplicated all appear to have `dep_types = NULL`,
and digging into this further, this is due to SQLite treating `NULL`s
differently in `UNIQUE` constraints[0].

However, this doesn't seem to be super straightforward, as we want to
have the `ON CONFLICT` in the schema, but that's not possible in a
standalone index[1].

One option is a partial index[2], but as we use this as a JSON array - and
we already have some handling for (de)serialising it when `NULL` - we
can simplify this and replace it with it always being an empty array.

This is a breaking change, as we are changing the underlying schema for
a table that may be relied upon.

Closes #88.

[0]: https://stackoverflow.com/a/47522530
[1]: https://stackoverflow.com/a/51938871
[2]: https://stackoverflow.com/a/68215933